"Oh, have a heart," Miriam said, and her own suit lit up. "Just for a time. Let him get used to it."

I said, "Get used to what? Falling into this endless dark?"

"Not endless," Poole said. "The ocean is no more than how much, Harry?"

"250 kilometers deep," Harry said, mercifully not presenting a Virtual to us. "Give or take."

"250... How deep are you intending to take us, Poole?"

"I told you," Michael Poole said grimly. "As deep as it takes. We have to retrieve that GUT engine, Emry. We don't have a choice simple as that."

"And I have a feeling," Miriam said bleakly, "now we're out of that vent, that we may be heading all the way down to the bottom. It's kind of the next logical choice."

"We'll be crushed," I said dismally.

"No," Harry Poole piped up. "Look, Jovik, just remember t.i.tan isn't a large world. The pressure down there is only about four times what you'd find in Earth's deepest oceans. Five, tops. Your suit is over-engineered. Whatever it is that kills you, it won't be crushing."

"How long to the bottom, then?"

Harry said, "You're falling faster than you'd think, given the viscosity of the medium. That spider is a strong swimmer. A day, say."

"A day!"

Miriam said, "There may be sights to see on the way down."

"What sights?"

"Well, the tube-fish can't exist in isolation. There has to be a whole ammono ecology in the greater deeps."

My imagination worked overtime. "Ammono sharks. Ammono whales."

Miriam laughed. "Sluggish as h.e.l.l, in this cold soup. And besides, they couldn't eat you, Jovik."

"They might spit me out after trying." I tried to think beyond my immediate panic. "But even if we survive-even if we find our d.a.m.n GUT engine down there on the ice how are we supposed to get back?"

Poole said easily, "All we need to do is dump our ballast and we'll float up. We don't need to bring up the GUT engine, remember, just use it to recharge the suits."

Miriam said, "A better option might be to hitch a ride with another spider."

"Right. Which would solve another problem," Poole said. "Which is to find a cryovolcano vent to the surface. The spiders know the way, evidently."

Harry said, "And even if the spiders let you down, I could guide you. I can see you, the vent mouths, even the GUT engine. This neutrino technology was worth the money it cost. There's no problem, in principle."

At times I felt less afraid of the situation than of my companions, precisely because of their lack of fear.

Miriam fetched something from a pack at her waist, I couldn't see what, and glanced at Poole. "Novak's not going to survive a descent lasting a day. Not in the dark."

Poole looked at me, and at her. "Do it."

"Do what?"

But I had no time to flinch as she reached across, and with expert skill pressed a vial into a valve in the chest of my exosuit. I felt a sharp coldness as the drug pumped into my bloodstream, and after that only a dreamless sleep, cradled in the warmth of my cushioned suit.

So I missed the events of the next hours, the quiet times when Poole and Miriam tried to catch some sleep themselves, the flurries of excitement when strange denizens of t.i.tan's ammono deep approached them out of the dark.

And I missed the next great shock suffered by our strange little crew, when the base of t.i.tan's underground ocean, an ice floor 300 kilometers beneath the surface, at last hove into view. The strange landscape of this abyssal deep, made of folded high-pressure ices littered by bits of meteorite rock, was punctured by vents and chasms, like an inverted mirror image of the crust far above us. And the spider we rode did not slow down. the spider we rode did not slow down. It hurled itself into one of those vents, and once more its limbs began to clatter down a wall of smooth rock-ice. It hurled itself into one of those vents, and once more its limbs began to clatter down a wall of smooth rock-ice.

Harry warned Miriam and Poole that this latest vent looked as if it penetrated the whole of this inner layer of core-cladding ice Ice VI, laced by ammonia dihydrate a layer another 500 kilometers deep. At the base of this vent there was only t.i.tan's core of silicate rocks, and there, surely, the spiders' final destination must lie.

There was nothing to be done but to endure the ride. It would take perhaps a further day. So Poole and Miriam allowed the spider to drag us down. More tube-fish, of an exotic high-pressure variety, grazed endlessly at the icy walls. Miriam popped me another vial to keep me asleep, and fed me intravenous fluids. Harry fretted about the exhaustion of our power, and the gradual increase of pressure; beneath a column of water and ice hundreds of kilometers deep, we were approaching our suits' manufactured tolerance. But they had no choice to continue, and I, unconscious, had no say in the matter.

When the ride was over, when the spider had at last come to rest, Miriam woke me up.

I was lying on my back on a lumpy floor. The gravity felt even weaker than on the surface. Miriam's face hovered over me, illuminated by suit lamps. She said, "Look what we found."

I sat up. I felt weak, dizzy-hungry. Beside me, in their suits, Miriam and Poole sat watching my reaction. Then I remembered where I was and the fear cut in.

I looked around quickly. Even by the glow of the suit lamps I could not see far. The murkiness and floating particles told me I must be still immersed in the water of t.i.tan's deep ocean. I saw a roof of ice above me not far, 100 meters or so. Below me was a surface of what looked like rock, dark and purple-streaked. I was in a sort of ice cavern, then, whose walls were off in the dark beyond our bubble of light. I learned later that I was in a cavern dug out beneath the lower icy mantle of t.i.tan, between it and the rocky core, 800 kilometers below the icy plains where I had crash-landed days before. Around us I saw ice spiders, toiling away at their own enigmatic tasks, and bits of equipment from the gondola, chopped up, carried here and deposited. There was the GUT engine! My heart leapt; perhaps I would yet live through this.

But even the engine wasn't what Miriam had meant. She repeated, "Look what we found."

I looked. Set in the floor, in the rocky core of the world, was a hatch.

XIII Hatch

They allowed me to eat and drink, and void my bladder. Moving around was difficult, the cold water dense and syrupy; every movement I made was accompanied by the whir of servomotors, as the suit labored to a.s.sist me.

I was rea.s.sured to know that the GUT engine was still functioning, and that my suit cells had been recharged. In principle I could stay alive long enough to get back to the Hermit Crab. Hermit Crab. All I had to do was find my way out of the core of this world, up through 800 kilometers of ice and ocean... I clung to the relief of the moment, and put off my fears over what was to come next. All I had to do was find my way out of the core of this world, up through 800 kilometers of ice and ocean... I clung to the relief of the moment, and put off my fears over what was to come next.

Now that I was awake, Michael Poole, Miriam Berg, and Virtual Harry rehea.r.s.ed what they had figured out about methane processing on t.i.tan. Under that roof of ice, immersed in that chill high-pressure ocean, they talked about comets and chemistry, while all the while the huge mystery of the hatch in the ground lay between us, unaddressed.

Harry said, "On Earth 95 percent of the methane in the air is of biological origin. The farts of animals, decaying vegetation. So could the source be biological here? You guys have surveyed enough of the environment to rule that out. There could in principle be methanogen bugs living in those ethane lakes, for instance, feeding off reactions between acetylene and hydrogen, but you found nothing significant. What about a delivery of the methane by infalling comets? It's possible, but then you'd have detected other trace cometary gases, which are absent from the air. One plausible possibility remains .. ."

When t.i.tan was young its ammonia-water ocean extended all the way to the rocky core. There, chemical processes could have produced plentiful methane: the alkaline water reacting with the rock would liberate hydrogen, which in turn would react with sources of carbon, monoxide or dioxide or carbon grains, to manufacture methane. But that process would have been stopped as soon as the ice layers plated over the rock core, insulating it from liquid water. What was needed, then, was some way for chambers to be kept open at the base of the ice, where liquid water and rock could still react at their interface. And a way for the methane produced to reach the ocean, and then the surface.

"The methane could be stored in clathrates, ice layers," Harry said. "That would work its way to the surface eventually. Simpler to build vents up through the ice, and encourage a chemoautotrophic ecosystem to feed off the methane, and deliver it to higher levels."

"The tube-fish," I said.

"And their relatives, yes."

Looking up at the ice ceiling above me, I saw how it had been shaped and sc.r.a.ped, as if by lobster claws. "So the spiders keep these chambers open, to allow the methane-creating reactions to continue."

"That's it," Michael Poole said, wonder in his voice. "They do it to keep a supply of methane pumping up into the atmosphere. And they've been doing it for billions of years. Have to have been, for the ecologies up there to have evolved as they have the tube-fish, the CHON sponges, the silanes. This whole world is an engine This whole world is an engine, a very old engine. It's an engine for creating methane, for turning what would otherwise be just another nondescript ice moon into a haven, whose purpose is to foster the life forms that inhabit it."

"And where there is an engine, there are engineers," Miriam said.

"Yes. But why did they build all this?"

None of them could answer that.

"Ha!" I barked laughter. "Well, the why of it is irrelevant. The spiders are clearly sentient or their makers are. You have found precisely what you were afraid of, haven't you, Michael Poole? Sentience at the heart of t.i.tan. You will never be allowed to open it up for exploitation now. So much for your commercial ambitions!"

"Which you were going to share in," Harry reminded me, scowling.

I sneered. "Oh, I'd only have wasted the money on drugs and s.e.x. To see you world-builders crestfallen is worth that loss. So what's under the hatch?"

They glanced at each other. "The final answers, we hope," Michael Poole said.

Miriam said, "We've put off looking under there until we brought you round, Jovik."

Poole said, "We've no idea what's under there. We need everybody awake, ready to react. We might even need your help, Emry." He looked at me with faint disgust. "And," he said more practically, "it's probably going to take three of us to open it. Come see."

We all floated through the gloopy murk.

The hatch was a disc of some silvery metal, perhaps three meters across, set flush into the roughly flat rocky ground.

s.p.a.ced around its circ.u.mference were three identical grooves, each maybe ten centimeters deep. In the middle of each groove was a mechanism like a pair of levers, hinged at the top.

Michael said, "We think you operate it like this." He knelt and put his gloved hands to either side of the levers, and mimed pressing them together. "We don't know how heavy the mechanism will be. Hopefully each of us can handle one set of levers, with the help of our suits."

"Three mechanisms," I said. "This is a door meant to be operated by a spider, isn't it? One handle for each of those three big claws."

"We think so," Miriam said. "The handles look about the right size. We think the handles must have to be worked simultaneously-one spider, or three humans."

"I can't believe that after a billion years all they have is a clunky mechanical door."

Poole said, "It's hard to imagine a technology however advanced that won't have manual backups. We've seen that the spiders themselves aren't perfect; they're not immune to breakdown and damage."

"As inflicted by us." I gazed reluctantly at the hatch. "Must we do this? You've found what you wanted or didn't want. Why expose us to more risk? Can't we just go home?"

Miriam and Michael just stared at me, bewildered. Miriam said, "You could walk away, without knowing?"

Poole said, "Well, we're not leaving here until we've done this, Emry, so you may as well get it done." He crouched down by his handle, and Miriam did the same.

I had no choice but to join them.

Poole counted us down: "Three, two, one."

I closed my gloved hands over the levers and pushed them together. It was awkward to reach down, and the mechanism felt heavy; my muscles worked, and I felt the reaction push me up from the floor. But the levers closed together.

The whole hatch began to vibrate.

I let go and moved back quickly. The others did the same. We stood in a circle, wafted by the currents of the ammonia sea, and watched that hatch slide up out of the ground.

It was like a piston, rising up one meter, two. Its sides were perfectly smooth, perfectly reflective, without a scuff or scratch. I wondered at how old it must be. Michael Poole, fool that he was, reached up a gloved monkey-curious hand to touch it, but Miriam restrained him. "I'd like to measure the tolerances on that thing," he murmured.

Then the great slab, around three meters wide and two tall, slid sideways. Poole had to step out of the way. The sc.r.a.pe across the rough rock ground was audible, dimly. The shift revealed a hole in the ground, a circle and at first I thought it was perfectly black. But then I saw elusive golden glimmers, sheets of light like soap bubbles; if I turned my head a little I lost it again.

"Woah," Harry Poole said. "There's some exotic radiation coming out of that hole. You should all back off. The suits have heavy shielding, but a few meters of water won't hurt."

I didn't need telling twice. We moved away towards the GUT engine, taking the light with us. The hole in the ground, still just visible in the glow of our suit lamps, looked a little like one of the ethane lakes on the surface, with that metallic monolith beside it. But every so often I could make out that elusive golden-brown glimmer. I said, "It looks like a facet of one of your wormhole Interfaces, Poole."

"Not a bad observation," Poole said. "And I have a feeling that's exactly what we're looking at. Harry?"

"Yeah." Harry was hesitating. "I wish you had a better sensor suite down there. I'm relying on instruments woven into your suits, internal diagnostic tools in the GUT engine, some stray neutrino leakage up here . .. Yes, I think we're seeing products of stressed s.p.a.cetime. There are some interesting optical effects too-light lensed by a distorted gravity field."

"So it's a wormhole interface?" Miriam asked.

"If it is," Poole said, "it's far beyond the clumsy monstrosities we construct in Jovian orbit. And whatever is on the other side of that barrier, my guess is it's not on t.i.tan .. ."

"Watch out," Miriam said.

A spider came scuttling past us towards the hole. It paused at the lip, as if puzzled that the hole was open. Then it tipped forward, just as the spider we rode into the volcano had dipped into the caldera, and slid head first through that sheet of darkness. It was as if it had fallen into a pool of oil that closed over the spider without a ripple.

"I wouldn't recommend following," Harry said. "The radiations in there are deadly, suit or no suit; you couldn't survive the pa.s.sage."

"Lethe," Michael Poole said. He was actually disappointed.

"So are we done here?" I asked.

Poole snapped, "I'll tell you something, Emry, I'm glad you're here. Every time we come to an obstacle and you just want to give up, it just goads me into trying to find a way forward."

"There is no way forward," I said. "It's lethal. Harry said so." no way forward," I said. "It's lethal. Harry said so."

"We can't go on," Miriam agreed. "But how about a probe? Something radiation-hardened, a controlling AI with luck we could just drop it in there and let it report back."

"That would work," Poole said. Without hesitation the two of them walked over to the GUT engine, and began prying at it.

For redundancy the engine had two control units. Miriam and Poole detached one of these. Containing a sensor suite, processing capabilities, a memory store, it was a white-walled box the size of a suitcase. Within this unit and its twin sibling were stored the ident.i.ty backups that had been taken of us before our ride into t.i.tan's atmosphere. The little box was even capable of projecting Virtuals; Harry's sharp image was being projected right now by the GUT engine hardware, rather than through a pooling of our suits' systems as before.

The box was small enough just to be dropped through the interface, and hardened against radiation. It would survive a pa.s.sage through the wormhole though none of us could say if it would survive what lay on the other side. And it had transmitting and receiving capabilities. Harry believed its signals would make it back through the interface, though probably scrambled by gravitational distortion and other effects, but he was confident he could construct decoding algorithms from a few test signals. The unit was perfectly equipped to serve as a probe through the hatch, save for one thing. What the control box didn't have was intelligence.

Michael Poole stroked its surface with a gloved hand. "We're sending it into an entirely unknown situation. It's going to have to work autonomously, to figure out its environment, work out some kind of sensor sweep, before it can even figure out how to talk to us and ask us for direction. Running a GUT engine is a pretty simple and predictable job; the AI in there isn't capable of handling an exploration like these."

"But," I said, "it carries in its store backups of four human intellects-mine, dead Bill, and you two geniuses. What a shame we can't all ride along with it!"

My sarcasm failed to evoke the expected reaction. Poole and Miriam looked at each other, electrified. Miriam shook her head. "Jovik, you're like some idiot savant. You keep on coming up with such ideas. I think you're actually far smarter than you allow yourself to be."