I

The planet was more than two light years in diameter, and a cautious 20,000 light years away its brilliance suggested a quasar. Even Sirius in the skies of distant Earth did not shine more brightly.

My lens-field magnified the star-like image into a planetary disk, basking in the orange light of invisible suns. In the infrared it showed a temperature of only 165K, which was a little cool for a planet, I thought. Who would want to live on it?

The spectrometer could find no chlorophyll absorption bands, hardly surprising at that temperature. Why would anyone build a planet without life? But perhaps the Builders were low-temperature life-forms that didn't use chlorophyll.

The spectrometer found interstellar hydrogen easily enough, but complained about anomalous Doppler broadening, and refused to recognise anything else. So the gas was in orbit. About a massive compact object. Big deal.

It's true what they say about characters like me — we don't seem to have much imagination. Perhaps you need a lack of imagination to face up to intergalactic journeys and alien artifacts without bugging out. Still, once an idea has been hammered into my brain I can analyse with the best of them.

A supramundane planet that big would have to be built around a black hole. An enormous black hole. A black hole that would give the planet an escape velocity not far below the speed of light and cause a marked redshift in the light that clawed its way up to infinity from the planet's surface.

I soon nailed it down; the redshift was 0.6976, a factor of 1.6976 in wavelength. There was chlorophyll a-plenty, and the surface temperature now came out at a pleasant 290K. I corrected the screen image for the redshift and saw a blue-white globe. It looked like Neptune. I was a little surprised, I'd been expecting it to look like Earth. Stupid of me. At this range, Earth would have been an invisibly small speck.

The redshift gave me a figure for the object's mass, about 2x10¹² solar masses. Not unreasonable for a huge cluster-dominant elliptical galaxy, and it neatly explained the dynamics of the nearby galaxies.

A black hole of that mass has a diameter of one-and-a-half light years at the event horizon (r=2M); the surface of this planet was only a little beyond r=3M, the radius of the last stable orbit, where light can only just circle the hole without spiralling in. I wondered why the Builders had cut it so fine.

I enlarged the lens-field and zoomed in on the globe, while the spectrometer flicked out id's on the atmosphere, biosphere and geosphere. Earthlike, all of them. At a magnification of 10¹² there were finally signs of surface structure; the globe began to look more and more like Earth.

Above the planet, strange structures circled. In them, the planet's suns were housed, hidden from above, lighting the world beneath. Along the horizon, stray flashes of sunlight peeped out into space.

An artifact abandoned after the Omega Point, perhaps?

II

Two years before, astronomers in the Epsilon Eridani system had discovered an anomaly. Nothing special, just a peculiarity in the motions of a cluster of galaxies far over the curve of the universe, at a redshift of 0.9. Either the cluster was unbound, or an invisible galaxy lurked in the centre; an oddity at a redshift of 2.2 was all even the largest lens-field telescope could detect.

The astronomers weren't worried; gravitational redshift would explain everything, except how so great a black hole could swallow a whole galaxy without becoming a quasar. They weren't interested enough to go out all that way themselves — billions of light years there and back — and the whole thing might have been filed away and forgotten if the military hadn't gotten interested.

I was the obvious choice for the job. They sent me out in a brand new 10m diameter faster-than-light sphere to find out what was going on. And if I met up with hostile aliens? "Don't let them capture you! Don't lead them back to Earth!" You see, even then the military thought the effect might be artificial, which shows they were wiser than the scientists, who were still inclined to view everything in terms of a dead universe.

Even after I'd found the supramundane planet, I was puzzled by it. You see, since the gravitational anomaly of this object had been detected as far away as Earth, it must have been built over ten billion years ago. And somehow, this place just didn't look that old. I had the feeling that it was actually rather new, unused even.

Perhaps the absence of interstellar debris had something to do with it. Over ten billion years, some stars should have wandered along from intergalactic space, even from the other galaxies in this cluster. Not many, perhaps, if the Builders had cleared out all mass for a million light years, but at least a few. Apart from some harmless HII regions, there was nothing. It was all too neat.

The radio spectrum was quiet, but the flux of neutrino emissions was extremely powerful — megawatts per square metre of surface. I could detect no modulations. Innumerable exabaud spread-spectrum channels might have been hidden in the noise, but I was pretty sure no one was trying to contact me.

Hanging around out here wasn't going to do any good. I engaged the drive and slid in closer, storing the potential energy in my relativistic string battery. The ship's relativistic mass increased, the universe brightened with blueshift.

III

I suppose I blundered. I slid down towards the surface, prudently staying well clear of the orbital structures. Neatness and symmetry is my passion, so I directed the ship precisely halfway between two of the suns. That was stupid, I would have done better to go at it bull-headed. I know that now. At the time I was proud of my careful piloting.

Precisely as I flew, I flew precisely into a stream of small black holes. None of the miniholes actually struck the ship, but later I calculated that the stream must have passed within a hundred metres. The tide ripped the ship apart and hurled it along the stream. It nearly ripped me apart too.

The control panel and I went one way. The string battery went another. Bits and pieces went all over the place, some of them impacting miniholes and drenching space with an uncomfortable amount of radiation. I tried to get the remnants of my ship to chase after the battery, but it wouldn't obey. I realised how fortunate that was when a teraton explosion blazed out across the sky.

I fell from a height of 3x10¹²m. In ten days I would hit the ground at 2.5% of the sped of light. I am not given to despair, but without the energy in my string battery, I was helpless.

Deus ex machina saved me. Three streams of miniholes shot up in a triangle from the surface, made gothic arches kilometres overhead, and fell back. A hexagonal fountain formed around me, on six slender spider-legs. It bore me up with its gravity and stayed with me all the way to the ground.

Okay, it was just an automatic system to protect the planet's surface from impact. I was still impressed; it would have been far easier simply to have blasted me and my ship into vapour. I was encouraged to hope that the Builders would be friendly. Or polite to visitors, at least.

IV

So here I am, on the skin of the great World. My ship is useless, my mission in shambles. All I have left is hope, the hope that I may yet meet the Builders of this world. With difficulty, I cut myself free and clamber down to the ground.

I have landed in a grove of apple trees. Yes, apple trees, or something so close I can't tell the difference. They smell like apple trees, even, though smell is not my most reliable sense.

The grass underfoot is … grass. The sky overhead is blue, the clouds are white and fluffy, the air is clean and pure and a terrestrial oxygen-nitrogen mix. There are birds on the wing, and a thrush is singing in the trees, and a rabbit scampers across the meadow.

Surely, this is no multi-billion-year-old alien artifact. It could be Earth, or any one of a million of Man's space habitats. Except for the emptiness. No towns, no cities, no signs of human life. No roads, no buildings, nothing but the green grass and the wildlife.

The multiplicity of suns isn't very Earthlike, though. One bright sun shines almost overhead, but down around the horizon there are dozens more. In the west they are red; in the east they are blue. The horizon itself is a ring of light and infinitely distant, here where the downwards bending of the light lifts the curved world up into a flat plane; and the jagged sky's edge is the fractal relief of hill and plain and valley.

I wonder how to attract attention, I wonder if there is anyone here to attract, here on this world 2.5x10¹⁸ times the area of Earth. If anyone lives in all this empty wilderness, he might be millions of kilometres, even hundreds of astronomical units, distant. Yet the landscape shows a taming hand in the neatness of the apple grove and the beauty of the woods and meadows and the rivers running by. This is no world that ran to seed a billion years ago. Surely, at the least, there is still a Caretaker.

I walk north along the golden mean of the sky, between red west and blue east, to where rocky mountains bare the bones of the land. As ever, the mountains are further than they look, but I have cast a cloud of self-propelling radar motes around me and am not deceived.

Evening brings a strange sunset before I reach the mountains. Blue suns in the eastern sky shrink to stars and vanish. The sun above, and the warm suns around it, gather towards the western sky; they dim, until the first sun has gone out and only those on the horizon remain. Reddened by their motion and their paths through the atmosphere, they rest awhile, then sink into the haze. Night falls.

After dark, I watch the strange night sky through the clear air. Not a star, not a planet. A thousand galaxies blaze alone, brightened by blueshift as their light falls down the potential well. Not so harshly blue as I expected; blueshift must bring as many infrared stars into the visible as it makes red and yellow stars blue.

In the Milky Way, even in the intergalactic gulf, the view of the galaxies is confused by foreground stars and nebulae. Here the galaxies are naked. It is a lonely place — no other heavenly object within a million light years.

Absurd. I am on an object which is a dozen galaxies rolled into one. How can it be lonely? And if there are no people here, I do not need them; there are other and far friendlier creatures.

I walk on through the night. The fixed galaxies shine down. The planet does not spin. Nought counts the passage of the hours. No moon rises. The darkness does not change. I walk on, towards the dim distant outline of the mountains.

A paling of the eastern sky heralds the unearthly dawn. A dawn that fills the east with a blue light, brighter than the red sunset, and colder. A get-up-and-go glow that splinters into diverse suns; blue-white suns burning through the white mist; and a wide-awake yellow sun bursting forth already halfway up the sky.

All over the rolling grassy plain, among the gorse and scrubby bushes, birds sing, bees buzz and insects hum. Rabbits make their early morning meal; weasels slink home to bed.

Far to the north, the mountains rise unchanging, blue with the distance and the eastern glow, gold with the risen sun. I walk on. The day wears on. Night comes again: sunset to sunset, a twenty-four hour day. Another dawn, and the mountains are before me.

On a plateau of bare solid rock and thin soil: the granite is old, weathered, crossed by tiny gullies of long-ago water; the rock, solid and natural, the bones of an ancient planet. I wonder. But I no longer have the equipment to measure isotope ratios and radon content. Even if I had, what would it prove? The Builders could surely have made young rock seem old.

I reach the bare rise in the centre of the plateau and set off a tiny explosive charge, a minuscule string battery. I listen for the echoes and record them for analysis. The first half minute is full of reflections from the mountains.

After 39.6 seconds I hear a powerful echo, weakly repeated 39.6 seconds later still. The solid geosphere is about 100 kilometres deep. I am impressed. Ten thousand times thicker than the crust of the usual space habitat, even three times thicker than the solid crust of Earth.

But this is no commonplace supramundane planet. It is a giant, a fraction over one light year in radius, its proper circumference 6.6 light years and its surface area nearly 14 square light years, astronomically greater than the area of all of Man's habitats put together. Yet if this world is like our own far smaller suprajovian and suprastellar planets, there will be a hollow base beneath the solid geosphere. And if the Builders or their Caretakers remain, it is there I shall find them.

I correlate the seismic echoes into a three-dimensional picture, learning of numerous passages and caverns beneath the surface. The entrance to the nearest is less than five hundred metres distant. I walk over to the tunnel mouth. Even though I know there is a door in the rock face, I cannot see it.

Feeling foolish, I say, "Open Sesame!"

The rock door swings open, revealing a smooth well-lit tunnel. Odd how the door seemed to understand English. But perhaps it analyses meaning in some cunning pseudo-telepathic way. Like, if someone stands in front of a door and yells, then they want to come through.

I risk pushing the door shut. It sucks itself the last millimetre to a hermetic seal, and becomes invisible again.

"Open!"

It doesn't move. I feel a touch of panic and cut short my experiment.

"Open sesame!"

Smoothly, it re-opens. It really does understand English. What's more, it reads the classics. From ten billion years ago? Ridiculous.

I shrug and enter the tunnel.

V

I am now deep below the surface of this world. I have climbed down half a million steps, I have traversed five hundred kilometres of corridor, passed through a thousand empty halls, ignored or briefly explored a thousand side passages. I have descended a hundred kilometres into the planet and found nothing. I am near the bottom of the deep crust, about to enter the geosphere base. If it is not a dead end. It has taken me two weeks to get this far.

At last the stairway comes out onto a ledge in a great cubic cavern, 9.8 kilometres on a side, according to my radar motes. A white mist seems to cover the featureless pearly floor.

Four microscopic stairways cling to the cavern walls; they start at each of the upper corners, end on the opposite lower corners. A thick transparent wall guards the outer edge, and every 100m is a landing 50m long.

I stop at the first landing and tape seismic microcharges to the wall. The reflection from the planet's surface is weak; the main response comes from the bottom of the geosphere, a further thousand kilometres below.

The geosphere base is an endless square array of 10km boxes, with 200m thick diamond walls. The floor is 200m deep diamond too. Impressive. But necessary, to hold up the immense weights of the geosphere crust. Going down, the walls in each successive layer are 1m thicker; at the bottom they are a full 300m. A hefty, crude, simple, yet basically elegant design.

At the cavern's roof, the air is thin, like the top of a mountain; a slow MHD wave along the tunnel has held back the downwards flow. Since the field has little effect on solid objects — less than a milligee — I have reconfigured to ignore it. Now although underground caverns and white boxes and mountain peaks should feel cold, the air is disconcertingly warm. It thickens naturally as I descend and at floor level is even thicker than on the surface.

I follow another stairway down through the floor, as well-lit and empty as all the rest. Do all the lights stay on for ever, I wonder? The cooling load would be considerable.

Down, down, I go: step by step; stair by stair; room by room; another thousand kilometres into the depths of the planet. And all the rooms are empty. And all the passageways are quiet.

VI

I have reached the bottom of the geosphere base. The staircase winds down a vertical shaft only five metres across through a final kilometre of solid diamond, and ends in a transparent gondola jutting out below the geosphere.

The first thing I see is the light of another planet laid out below. I cannot judge how far it is: one moment it seems near enough to touch; the next it seems as remote as the stars. I suspect the latter. I send out a radar signal and wait for its return.

If that featureless blue-white planet is Earthlike, it must be at least an astronomical unit distant. I wait many hours, but the radar signal does not return. I adjust my sight to spectrometer mode and correlate against redshift. Ten percent. With gravity a little under 1g that puts the gap at a tenth of a light year.

The void between the shells is almost empty. I look for the orbital rings supporting the planet above me; their mass and bulk should have been about as great as the geosphere, filling the anti-sky with a dense criss-crossing web. Instead I see a ghostly filigree of finest gossamer lines, with tiniest glimmering sparks at the junctions of the web.

I trace the lines and see that they form vast arches, opening out into hexagonal domes. The six-fold arches merge into a hexagonal array of tapering columns, 10¹¹m apart. Despite the immense weight on each, the columns narrow to indetectable thinness as they approach the planet below. Streams of miniholes press up on the underside of the geosphere and curve back down again, a spider's web of momentum flow on six-fold spidery legs. It makes sense, since you can throw miniholes around at relativistic speeds without worrying about tearing them apart. And it saves a lot of space.

Some of the thin light must be Hawking radiation from the miniholes themselves. But most must come from random molecules of the interplanetary medium, scattered off the minihole streams, a negligible power loss in that titanic flow. And the dew drops glistening on the web must be the way stations that guide, hold and divert the minihole streams.

Sedately orbiting through the arches, perhaps one to every dozen domes, are stars, clothed with suprastellar planets of their own, vast but miniature versions of this whole great structure.

There is another thin visible web amongst the miniholes, a thin web of ordinary matter. A transport web, whereby cargoes and … men? … can travel swiftly and easily between the planets. The momentum loop lines I can see, even some issuing from holes in the base close by, but there are no vehicles traversing them.

From the gondola where I stand, a stairway in a crystal tube sweeps out and down into the toposphere, a stairway that spirals down round and round the minihole column to the surface below, a stairway it would take me thirty million years to descend. Why should the Builders have built such a stairway?

In the two months before my radar signal returns I search for an entrance to the transport web. I do not find one. So how can I get down to the next planet? I do some calculations. If I simply leap out and fall free, I will reach the surface in about five months (proper time) and impact with the energy of a 250MT bomb. This will cause significant damage to the planetary surface. Therefore it will not be permitted. Therefore a minihole fountain will catch me and let me gently down to the ground. All I have to do is leap.

The logic is inescapable. What a marvellous parable it will be — if I survive. A leap of faith. Or am I anthropomorphising the Builders unjustifiably? But I don't have many other options, except to go back to the surface and forget my mission. I'm tempted, but not greatly. I may be low on imagination, but I'm hot on duty.

I hesitate a long while before taking the plunge. Once I start, there will be no turning back.

I scream and jump.

VII

My fears were groundless. I stand again on the surface of an Earthlike planet. If the sky comes to an end beneath the shell of the upper planet, the eye cannot tell. Stars and galaxies wheel in their course, and in the day a single sun tracks from east to west. None of them real, of course, merely projections from the roof. But they look real. It's a standard trick.

The climate here is pleasant, the landscapes attractive, the wildlife abundant. And like the outer planet, it is empty, uninhabited.

I measure the depth of the geosphere beneath me. Fifty kilometres. A deep crust, but only half the outer planet's. A series of nested planets, it seems, each half the thickness of the last. And the Builders, perhaps, a dozen levels down.

I find an entrance leading downwards. Five hundred and fifty kilometres below I reach the bottom of this planet's geosphere, and look down across an immense gulf to the third planet. Minihole arches hold up the globe. The redshift tells me the gap is only 5x10¹⁴m. Only.

As before, I search for an opening to the transport web and find none. As before, I have to trust to the safety engineering of the Builders and leap into space.

VIII

Here the diminishing series is complete: each shell in turn just half the thickness of the previous shell, the gap just half as wide. Now the crust is only an average of ten metres deep, upon a contoured geosphere base ten kilometres thick. Typical values for space habitats. They seem dreadfully thin.

I have seen no signs of the Builders, and realise I have only passed through the outermost layers of this world. For beneath me is still another shell, across a void of 1.1x10¹¹m, and the arches of miniholes still support the world. This planet too is blue-white as Neptune, and closer up will be as Earth.

I search as before, and as before I find the foolish staircase, and no access to the transport web. I am beginning to lose my nerve over jumping: will the Builders really hold me up, lest I dash my foot against a stone, all the way down to the core? My impact energy from this height would only be 25kT. When do they decide it's too small to matter?

Paralleling the staircase is a round frictionless tube. I have little imagination, but it takes little imagination to recognise a helter-skelter. A solar system sized helter-skelter, perhaps, but still a helter-skelter. Or perhaps I should call it an emergency escape chute. There is even a pile of mats near the entrance hole, the first moveable objects I have seen in these subterranean caverns.

If my speed is limited to the speed of sound it will take 30 years to get to the bottom. I hope this isn't a practical joke like the staircase. I have this image of an endless stream of long-dead desiccated corpses spilling out the bottom end.

Eventually, I try it. If it takes thirty years I'll know better next time. Besides, I can blast my way out if necessary and fall free to the planet in a day and a half. And hope another minihole fountain catches me.

Riding the helter-skelter is enjoyable at first, hours of smooth acceleration. The chute seems to carry a batch of air along down with me — some kind of MHD airlock arrangement. After a while it gets boring; the whole journey takes five days. I begin to get worried as the surface approaches at speeds upwards of a hundred kilometres per second, but the coils flatten out and the speed bleeds off — magnets in the mats, I think — and I am deposited safely at a walking pace at the bottom. A very neat design.

IX

I descend through an apparently endless succession of shells. Each is an Earthlike supramundane planet of 12 square light years, each separated from the next by about 10¹¹m. Each has ten metres of crust and ten kilometres of geosphere base, supported by minihole arches above the next. And on each time flows more slowly — by ten parts per million — than on the level above.

Each shell has its landscapes, all of them empty of intelligent life (so far as I can tell). On each, I look for the Builders, and find nothing. Beneath each shell is a transport web I cannot reach, a staircase I cannot use, a helter-skelter chute and the pile of mats. Sometimes I use the chute. Sometimes I leap off and let the minihole fountain set me down. Mostly I jump. It's quicker.

I count off planetary shells at the rate of one every three days. Occasionally, whenever I become too bored or lonely, I take longer, wandering about upon a planet's surface. I talk to the animals a lot, for they show little fear, even though they do not understand me.

On one planet, I journey for a whole year, a billionth of its circumference, and see perhaps 10^-21 of that globe. On another, a tiger springs out at me from a thicket; poor thing, it startles me and I strike too hard and kill it. That death haunts me through a hundred planets; maudlin of me, but the beasts are here my only friends.

Thousand by thousand, the shells mount and the years pass. And the helter-skelter and the fountain still bridge for me the void. And the world is still empty, and I am still alone.

X

At last, a change. Even from above this planet looks different, a whiter blue. As the minihole fountain carries me down, I see the city. A great city. A planetwide city. An endless city of eight square light years. The city of the Builders?

The fountain sets me down upon a mountain of great buildings that throng the slopes in ordered confusion, gleaming with crystal and gold. Towers, domes, arches, courtyards, cliffs, ledges, platforms, bridges, gossamer walkways in the sky. And in amongst them trees and gardens and hidden parks and pools and flowers, climbing, tumbling, spreading, blooming. And the scent of roses and the splashing of fountains and the gurgle of brooks.

And as far as the eye can see, buildings, buildings, buildings, of every size and shape, and amongst them a generous fractal patchwork of green parks, small parks, medium-sized parks, huge parks, parks the size of planets, even parks the size of a solar system, all surrounded by the city. And for some of the parks their own cities within them, and parks within the cities within the parks …. But all part of this one great City, surely the greatest city ever built.

And yet I see that the city is empty, the buildings unused, the streets untrod. Even the machines that tend the immaculate parks are not humanoid robots, but only clouds of microscopic motes. My radar motes may find them company; to me they are nothing.

A temple stand here — I have to call it a temple, it's far too big for a cathedral. It spreads its base over ten thousand square kilometres of ground, and lifts its pinnacle a thousand kilometres into space. It is the most magnificent building I have ever seen. And the most useless. It is almost too big for pilgrims or tourists to gawk and wonder at, and far too big to hold a service in.

And yet it is trivial, a microscopic pimple on the planet-city. The Builders of a World like this do not need such toys to show their skill or faith or dedication. Why build it then?

For that matter, why this city of many mansions? For whom has it been prepared? For Man? We can build our own. Great size is only size; when we have enough people, we shall build such worlds ourselves.

Disappointed, I leave the great city and go further down into the world, shell upon shell, planet upon planet of empty countryside.

XI

I have wondered what supports the world, the whole world. Yes, the minihole arches hold up each shell in turn, but what holds up the columns that carry the arches? … What stops the whole lot falling down the underlying black hole? An elephant … on the back of a turtle … on … what? It's an old problem.

In the Schwarzschild geometry, the last stable orbit for photons is at r=3M; everything closer spirals down through the event horizon at r=2M in a few orbits. So no suprahole planet can be dynamically supported any closer than r=3M. Yet even the outer layers of this world are no further out. It seemed then as if the minihole arches must ultimately reach, in some impossible fashion, right through the event horizon itself.

But now, many shells deeper, I can see the fallacy. Each planetary shell feels only the gravity of the mass inside it. This mass, M(r), can decrease faster than the radius r itself, so that of all the shells it is the outermost that comes nearest to its own gravitational radius. Indeed, there may be no black hole beneath this construct at all.

The gravity, which in geometrodynamic units obeys the equation g=(M(r)/r²)/(1-2M(r)/r), has remained at one gee from shell to shell. Going in, the perceived mass is increased by gravitational blueshift between levels, but drops back abruptly on crossing each shell. Clearly, the Builders have adjusted the spacing, down to 4.0x10¹⁰m at the bottom, to keep the gravity constant.

It is easy to see that r>3M here, because light rays in vacuum no longer curl around the globe and flatten it into a plate. With care, I can measure the actual curvature.

There are 111,000 levels in this sequence, and average of 7.0x10¹⁰m apart. At the bottom, times runs 3.8 times more slowly than in the universe outside. I have descended vertically 0.8 light years from an initial radius of 1.0 light years at the top, down to 0.5 light years now. General Relativity again.

I have journeyed for a thousand years, and still I have not ended, but the universe has aged 2.7 times as much. Where are the Builders now? And where is Man?

When I returned not home, other ships must surely have followed me, if not at once, then in a year, a decade, a century, a millennium. And of those ships, there must have been those that landed safely and returned home with the tidings. And other ships would have come to colonise and men spread out across the globe. And within two thousand years, even that vast land would surely have been tamed.

But I have travelled fast, in the arms of the minihole fountains. Those that follow me may be no faster. I have left messages, for they may be at my heels, just a few scant years or centuries behind. But there is a universe without, and they will have their own artifacts to create; they may be less eager than I to reach the core. But they will come. They will come.

XII

An uncharacteristically narrow gap of ten thousand kilometres gives onto an airless planet of metal below me. I cannot risk jumping down to it; there may be no minihole fountain to catch me. And there is no helter-skelter, only a stairway it takes me four months to descend. I say months, though there are no seasons here.

The planet is an endless library, room after room, ten kilometres in depth, light years in extent. Room after room along a space-filling curve is labelled sequentially with twenty four letters. Every room is filled with shelves, every shelf with books, all labelled on their spines to a further four letters, their texts commencing with the same letters, then degenerating into random gibberish. In one book I almost find a sentence that makes sense.

As I leave that level I wonder at the mentality of the Builders, putting so much effort into a silly literary joke. True, the library is but a millionth part of the whole artifact, and perhaps it was meant some day to become a real library. Indeed, for all I know vast oases of real libraries might already exist, dotted throughout the futile parable. Crazy, all the same.

Stars orbit through the arches in the 10¹²m void to the next planet and the next sequence: four thousand levels, through which the Schwarzschild r coordinate shrinks from r=4M(r) at the half-light-year point down to r=2.3M(r), and gravity increases to 10g. I reconfigure to cope, becoming shorter and stockier. Level by level, tall trees and large animals become more and more rare.

Another long sequence in at 10g pulls back from the black hole verge. Now the vegetation is all low-lying, the creatures small or prostrate. The animals that were my friends — the birds and mammals — are absent. Strange unearthly shapes replace them. The landscapes too are strange, horizons lifted up into a bowl by gravitational curvature of the light. I no longer waste time wandering about, but drop from shell to shell as fast as I can.

Now that I have little choice but to press on down to the very centre, I wish I'd remained in the one gee habitats. Yet now that I must reach the core, the core seems almost to retreat from me; though I pass through hundreds of thousands of shells, 1.4x10⁹m apart, the coordinate radius seems barely to decrease, so deformed is the geometry of space.

Now I am weary of this odyssey, and scarcely mark the passage of the shells. And so I descend into the depths. It is some slight relief when I pass through a surface of r=3M(r) again, and a true horizon reforms; but the land is too unearthly, even the sunlight is red, and the climate is at once too hot and too cold.

The lower levels are more and more like the circles of Hell: diffuse red light, no stars, close heat and icy coldness, stony ground and ice, the only life forms moss and lichens. Another literary joke, I presume. In the absence of the damned souls , it falls flat. Yet in its loneliness, this is a deadlier inferno than ever Dante dreamed. Can these levels truly be dungeons for the punishment of malefactors from the planets above? Or is this World so trivial to the Builders, that they must crack expensive jokes to amuse themselves?

Each million levels or so, the coordinate radius halves. After 29 million levels the sequence ends, the shells only 10km apart, the radius only 3x10¹⁰m. Now the gravity falls again to 1g through 2.7 million levels. At a radius of 3x10⁹m, the separation falls again.

Here all the frippery of life is gone. The shells are empty, bare smooth crystal on the outside, empty rooms within. Endless corridors and cold rooms, like a hospital; I can almost smell the disinfectant.

At a radius of 3x10⁸m the levels merge and I approach the core at last. For me, some 12,000 years have passed since I stood upon the surface and gazed at the galaxies; but in the world outside 18 million years have come and gone, stars have been born and died, and the Sun has made a twelfth of a circuit round the Milky Way.

XIII

And now in the very last shell of all I stand, and every 38 seconds another day passes by. No shells are there below this shell, only a void, across four thousand kilometres to the opposite side. And in the exact centre of the void, the exact centre of all this world, a point of light. A Planck wormhole, inflating gently into space, pouring out 10⁴³W (only a billionth of its potential luminosity) as high energy radiation first to be garnered by black holes and then to feed this whole unlikely edifice.

It is an entry into another space, but not for me; a doorway so small that even an electron could never pass it; a wormhole only 1.6x10^-35m in size; yet the core, the kernel, the foundation of all this mighty monument, pouring out life-giving power to all its myriad planets. No black hole, but the tiniest seed of infinitely convoluted space-time from which a whole universe could erupt, confined below the greatest planet that has ever been.

I have to admire the poetry, the gall, the chutzpah. But where are the Builders now? Not here? Not even at this the kernel of their creation? Then they must have left it long ago, and my search has been in vain. Yet it is not in me to rest, even if my task is done. If my golden fleece was a chimera, I must still turn about for home and the end of the odyssey; having found in the Holy Grail a fount of life, I must still return to Camelot.

I shall not stay here at the heart of the world, waiting for the end of time or the coming of another pilgrim. I shall retrace my steps, climb the foolish unclimbable stairway to the sky, though the Universe should end before I reach the top. For if I climb, I shall some day meet the men that follow; and I shall greet them and tell them of the kernel at the centre of all things. Then I shall be transported home; and I shall be at peace.

And some day, we shall follow the Builders where they have gone, and see the greater wonders of that Paradise, and build far greater wonders yet.

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