In the belly of the behemoth, her hand arced loosely over the nerve bundle, the diver switches off her lamp, and two meters of murky green visibility drops instantly to perfect black. The touch of her hand against the rail is her only landmark. She deftly fixes her wetsuit's tether there, careful not to put too much pressure along the nerve, and then stretches her burning limbs out into the thick invisible surrounds. Unseen tendrils brush over her calf as she does so, briefly tickling over the tactile suit material. She leaves her light off, ignoring the sensation. The message she carries is for the anthropology outpost, still two days' swim away; her battery life is precious. Besides, it can only be some dead thing.

Many people cannot stand to be alone this deep in the wreck, but Gwendolyn has always slept best here. It is part of what makes her suited to the odd profession that has befallen her—this endless running up and down the rails of nerves that plunge through the ship's great central depths. There is a certain self-abandonment in diving, a dropping of the mirror one must otherwise constantly point at oneself for the sake of others' comfort. Here there are no others. Here are only the endlessly drifting alien bodies, large and small, suspended in the vast algal matrix that had been meant to sustain them through the stars.

The ship is an ark filled with ocean; an egg that will never hatch.

It is not known which of the hundreds of species now entombed here originally built the vessel. Whoever the builders were, though, they were consummate bioengineers, with imaginations of an ecological scope. The ship has no electronics, and hardly any metal at all. Its hull is a thick, chitinous substance, extruded by some unknown bacterial process. Its wiring is all neurocircuitry—complex webs that run along and into the walls, miles-long axons that shoot across the central belly. The mechanism of propulsion is unknown, and evidently non-functional; it is difficult even to determine precisely when propulsion failed, whether the engine was ejected, whether perhaps no independent means of motion ever existed at all. Now nothing remains but remains, eerily inert, dead down to the tiniest decomposer.

The first day.

They have spent a long morning and afternoon securing the long-range transport to the alien hull, positioning the converted airlock, then letting the drills and probes do their work. These first moves are essential, the first readings, the first divers; given the remoteness of the alien vessel and the expense of the fuel, this single expedition is their only opportunity. The mission is seven years, including the eight months at either end, and nearly fifty scientists have committed to the long, cramped jaunt—marine and cell and neurobiologists, xenoanthropologists and xenolinguists, computer modeling specialists, astrophysicists, plus technical and medical and miscellaneous support—a whole university in a bottle, all carefully selected, vetted, pre-quarantined and implanted with contraceptives. The last two weeks have been taken up by petty politics and committee meetings over the first exploratory team. In the end, three are chosen: Aiden, for xenoanthropology; Zane, a marine biologist; and Gwendolyn, for the engineers.

"It seems I have to become alien to see aliens," jokes Aiden, as he latches his nutrient converter to the back of the hulking wetsuit. "Like Rich has it—'The absurd flippers, / the grave and awkward mask . . . '"

Behind his faceplate, Zane screws up his brows. "Hang on, hang on, I've got it—'Diving Into The Wreck'? Hm. A little on the nose, Aiden?"

Aiden touches a finger to his own nose. "A+."

"Out of the way, please," says Halah, the chief of medical, interrupting Aiden's arm to raise and fasten his mask.

Gwendolyn, already waiting by the hatch, suppresses a grin. Aiden has been making a nuisance of himself for the past two months, pulling up and excitedly sharing every bit of nautical poetry he can find in the skiff's databases in an attempt to confer cultural education upon the hard-science masses. By now half the expedition quotes "The Tempest."

"'Diving Into The Wreck' . . . more like diving into the graveyard," says Halah. "No—diving into the corpse. The giant, alien corpse. I'll stay in our nice, comfy skiff, thank you."

"What, for the whole five years?" Aiden grins. "Come on, there's plenty of suits for astronautical staff, too. Don't you want to see the undersea wonders, rich and strange?"

Oh, enough, Gwendolyn thinks, spare us from poets and anthropologists. With all that politicking, she's lately wondered if part of the motive behind Aiden's selection were to get him the hell off the skiff.

Then Zane's at it too, grinning at Gwendolyn, trying to pull her into the joke. "You afraid of a few alien ghosts, doc?"

In response, Halah only shakes her head, and finishes the checks on Aiden's suit. "Cleared," she says. "Stay in contact every ten minutes. Any wooziness, nausea, euphoria, anything strange, don't think twice about reporting it—you are required to report. If all goes well, we will see you in six hours for first check."

(Three hours later Aiden will crawl back up gasping and wild-eyed, stammering about the green algae that closes like a fist around you, half-glimpsed things that lurk around the edges of vision, of the sound of his own heartbeats pounding out into emptiness. Too much imagination. For Gwendolyn, it is different. The moment her mask plunges into the soft fluid, she feels a sense of quiet welcome.)

Deep in the ship, Gwendolyn wakes, takes a moment to relieve herself, then swims a little ways up the nerve bundle before detaching her suit's converter intake to check it over. There's no real hygienic reason for the distance. The suit takes care of all the filtration, incoming and outgoing, separating and processing the water, gasses and algal proteins that she takes in, and sterilizing her own waste; still, it feels wrong to eat what she shits. Her fingers find a number of tiny crustaceans have lodged in the filter intake, and their strangely perfect square carapaces catch on her gloves as she dislodges them. How they tend to congregate together, these little tiled creatures, even so long dead: relics of a lost biological algorithm. Gwendolyn reattaches the converter and waits for it to beep through its internal diagnostic checks. A sharp suck on her straw Then starts the flow of nutrient pap—the taste of the reprocessed algae is earthy and sharp, a bit like raw turnip. Still suckling, she switches on her lamp and kicks up through the water.

When the data systems had first gone down, as one of the more accomplished divers on the expedition, she had begun to serve as a courier for data exchanges between the air-bubble outposts. Her messages were drives in watertight bags, full of image files and raw data, notes on new findings; a few personal messages, no doubt the occasional love letter. Back then there had been more company on the dives. Technical and medical support staff often moved between the groups, and researchers did, too, with a courier as guide. The neurobiologists were especially peripatetic: either Yiwei or Anjali was always shunting between digital modeling and cell biology.

These days the messages are simpler, and rarer. It's mostly gossip: who's in love or out of it, who is sick, who is lost. Or missing. Many research groups have combined, abandoning site-specific research for the sake of what small community they can find, building little villages on the inner hull. Others have splintered away. The marine ecologists didn't take long to go completely nomadic: sometimes Gwendolyn cannot find them for months at a time, eventually coming across an old note left on a way-marker with their coordinates. The coordinates, too, are often by then out of date, sending her off to follow a trail of numbers across the depths.

The one piece of news they want to hear is something she never brings them. There is a cruel pattern to it. They always ask, as soon as she arrives, forcing her to articulate that "no," to face their serial disappointment. At first, she would swim into town already stiff with anticipation, unable to meet their welcomes in kind, steeling herself for the sting. After a time they had come to expect that coldness of her, and any sign of brightness that she gave would raise their hopes that the news would be different this time. It was kinder, in the end, to be cold. Now when Gwendolyn sheds her suit, she is already casting her imagination forward, back into the depths. Then the words bounce empty out of her mouth, a recitation without an audience, without signification.

No, there has been no contact.

No-one is coming to rescue us.

After the exploratory teams return, the researchers are cleared to set up on-site outposts. First there are two semipermanent bubbles, then five, then ten; the genetic molecule is located, the neural webs in the walls imaged and mapped and mapped again, genome after genome sequenced, tracing the footprints of the original engineers. The ecosystem is a library of genetic techniques, all fed into the main database on the skiff's computers, pinging a steady stream of findings back across space. The years tick by in other ways, too, in minor scuffs and wiring shorts in the equipment, battery faults and human health complaints—all the small, inevitable problems, foreseeable and unexpected alike. The damn water gets everywhere, corrosive and penetrating. The on-site bubbles work beautifully, just as designed; the modified airlock to the skiff, on the other hand, is a jury-rigged solution, and constantly under minor repair. It is an attempted maintenance of the antigravity system, three years into the expedition, that triggers the cascade. First a few errant drops in a heating element, then an acrid steam that sends Halah, Aiden, and the rest of the home-base core crew coughing and scrambling; a panel left open in the chaos; a door that will not close . . . 

Gwendolyn is working on the far side of the alien behemoth the day the skiff floods. It is Halah who later tells her about how slowly the water really crept in, in retrospect, and how quick it all seemed in the moment—a two-hour mad dash, salvaging batteries and medical necessities, candy bars and dry comforts, deck by deck with the water rising below. How Aiden could not bring himself to put on his mask, even as the water lapped at their chins, forcing her to sedate him. How the whole skiff fell silent, slowly, as the buzz of its systems died away one by one; and then, filling the wake of that mechanical silence, came the distant echoes of the inner sea.

Gwendolyn has been swimming for about six hours when a faint, faraway hint of illumination catches her attention. At first she thinks it is a trick of her eye, maybe the first sign of migraine aura—twenty years of living on converted alien algae has worn on all their bodies—but when she moves her head, the dim light maintains its relative position. The blackness is not entirely black today. There is a patch of gray in the green, twinkling across the water, like the light from a failing diver's lamp.

Halah.

Almost before she realizes it, Gwendolyn has sprung away from the rail. Light does not travel well in this algal greenout, so the source cannot be far—but then she checks herself, heart pounding, and comes to a full stop. Her hands shake as she switches on her lamp. She wheels around slowly until she spots the faint line of the axon, already barely visible just a few strokes away. The tether first. As she fixes it to the rail, her mind is reeling, calculating. Halah left the biologists twenty-one days ago on a solo dive down the long tertiary nerve to the main computing station, at which she never arrived. Gwendolyn has been coming across the eighth transverse—which is not so far, at this point in the swim, from biology; certainly within the realm of a drifting current. A wetsuit battery lasts three weeks, give or take, depending on use. If Halah had gone astray quickly—if she had then been quiet, cautious with her lamp, as Gwendolyn taught her to be—the battery could have a little life left. Enough to sustain a little life for the suit's occupant, in turn? For a trickle of oxygen, a flickering emergency flare?

Gwendolyn stops beside the rail. She shuts her own lamp off, forcing her breath to be calm and methodical as she waits for her pupils to adjust. The glint of green brightens. She dives toward it, and the tether plays out behind her.

The fickle light is steadier in the darkness, but the water is not empty. Not more than a few meters in, something large eclipses her vision, and before she can slow herself her hands are pushing into soft flesh, a broad stomach covered in little sticky fronds. It takes her a minute to extricate her hands from the pitcherfish, trying not to rip the sensory material from her gloves, or to push so hard that its wide, flat wings completely enfold her. When she finally shoots free, a cluster of long, hairlike tentacles, attached to another creature stuck to the pitcherfish's belly, entangle her tether, and she has to turn her lamp back on to unknot herself, temporarily ruining her dark vision. But when she is free, the illumination is back with her, stronger now, and without thinking she strokes out for it again.

How far has she come, how far drifted? The tether is still lax, although by her suit's clock, she might easily have swum a hundred meters, nearly its full length. She comes through a whole cloud of crustaceans, and her filter whirrs madly; cleaning it will have to wait. The light is not behaving the way she expected. It is broadening, not brightening, turning the thick water around her a muted green-gray. She can see the outline of her arms against it. She can see reflections glinting back at her from dead eyes and dead scales hidden deeper in the algae.

Ten meters later she sees the first jellyfish. It is no larger than a woman's wedding ring, its circular body fringed with glowing blue cilia that wave slowly in the current. All around her the light is coalescing into points, resolving into creatures, and then all at once she is in the leading edge of them: a huge cloud of bioluminescent creatures, extending for meters and meters all around her, like a candlelight vigil.

She cannot seem to stir. How had they all converged here, these unknown, uncatalogued creatures—and how had they remained undiscovered for twenty-one years? Had they been tiny intelligences, gathered together here for companionship in the face of the oncoming cataclysm? Or the bioengineered lamps of another race of creatures, massed together for one great illumination as the lives of the vessel winked out around them?

Halah's name is on her lips, falling out into the bright water.

Halah is clinging to the rail. She has her whole arm chicken-winged around it, her breath heavy and irregular. They are only twenty minutes out from the anthropology outpost.

"We can go back," says Gwendolyn. "I can bring one another one of the medics with me instead. It doesn't have to be you."

"No," says Halah, taking a deep breath. "If I tell myself that, it will be excuse after excuse, and I will never leave the outpost."

"You're out now," Gwendolyn says. "Why not just a quick swim up the hull and then back to anthro? It's only your first dive. It doesn't have to be a long one."

Halah shakes her head. "I'm going with you," she says. "If this is what our lives are now, I must take my life back. I must take care of my crew."

Gwendolyn masks a sigh. At this rate, they will never reach the xenolinguistics outpost.

Halah is one of the more senior members of the expedition, and no great swimmer, either. Her kicks have a lot of wasted motion, wearing her out quickly, and she compulsively grabs the axon at every stroke. How the woman ever made it from the skiff to the nearby anthro outpost—and with an unconscious Aiden in tow, too—is a mystery.

"I can hook my tether to you," Gwendolyn offers. "We've still got at least six hours til we hit the first way-marker. Then we'll tether over to transverse 35, sleep, and then it's a quick four hours up to xenolinguistics tomorrow."

"Sleep in the suits? I'd rather go ten hours straight through," Halah gives a nervous laugh, then a little shriek as she ducks to avoid a long, spiny stickstack tentacle. "Uh . . . are these guys common out here? Looks a bit dangerous."

"Not uncommon, when a diver kicks up a wake and sets the bodies moving," says Gwendolyn. "A fatigued diver can make mistakes. That's why we sleep. Here. I'm not going to tow you, it's just so you can follow."

She hands the hook of her tether back, forcing Halah to uncurl her arm, and then kicks off without waiting. She'll set a slow pace, just enough to keep the other woman occupied, to give her a chance to catch the rhythm of the stroke.

At the way-marker, Gwendolyn takes her tether back and fixes it to the pink ring. She measures out the thirty-eight meters, motioning Halah to follow, and begins to swim in a circle, casting around for the marker on the other axon. Halah follows silently, her breath coming hard. When they find the transverse and Gwendolyn releases her tether, Halah simply mimics her actions: set tether, clean filter, suck pap.

"How long does this axon bundle run?" she asks, after a few minutes.

"About thirty kilometers. The one we followed earlier is much longer. It cuts nearly all the way down the ship's long axis."

Halah shudders a little, staring at the rail where it vanishes into the limits of their field of vision, as if trying to play it out meter by meter in her imagination.

"Did you help with the mapping?" she asks. "You runners call them the 'railroad', right?"

"Just 'rails'." Gwendolyn corrects her with a smile. The first swim accomplished, she is inclined to be charitable. "And it wasn't like that, a big exploration. We used a sonar probe to map all the bulk structures before we ever dived, day one. You can call up the map in your heads-up display, if you want; the suit has some limited sonar for navigation, too. I did help program the suits to cross-reference with the map. It works like a GPS. Kind of."

". . . but it drains the power?" Halah finishes.

"Well . . . it doesn't always work," says Gwendolyn. "The big fish float around, the range is compromised by the algal density . . . but power-wise, the sonar itself is negligible compared to the suit's converters. Or even compared to the lamp. Speaking of which—you must be tired; lights out?"

Halah nods, but when Gwendolyn turns off her lamp, she leaves hers on a while, staring into the greenout. Her body is unilluminated, behind the corona of the lamp, like a stranger who comes up to a tent carrying a flashlight.

"If they don't drain the power, why not use the map?" Halah asks at last. "Is this some kind of bravado? Or you just don't need it?"

"Go to sleep, Halah."

"I wonder how long it will take me to know it so well?" Halah's voice falters, then resumes. "I can't always be sheep-dogged about, you know. I'm a doctor. I need to be able to make my rounds myself."

She trails off, but Gwendolyn does not open her eyes. They have a long swim tomorrow. After a time, the other light goes out.

Then, not long after, all at once it is on again and Halah is jerking in little spasms, breathing too fast, making a little noise in the back of her throat. Gwendolyn turns her own light on and quickly looks over Halah's body. Nothing is visually wrong—it is only panic, pure terror, but in a diving suit that is enough to be dangerous. The gas filter is not well-equipped to compensate for wild gasping.

"Halah, can you reach your arm out?" Gwendolyn says. "Come on now. Put your hand against ribs. Can you feel my breathing? All right? Can you breathe with me? Slow. Like this."

The hand clutches, then Halah pulls her close, her whole frame trembling as she struggles to control her breath. Gwendolyn tenses, startled, then forces her own breath to remain steady, setting the rhythm for them both. After a while, she puts her arms around Halah. When the gasping finally subsides, neither woman lets go.

"I'm so sorry," Halah says. "Something brushed against me. In the dark. I thought I could handle it, but—oh, god, I'm not a diver, I'm mission staff, I was never supposed to leave the ship—"

"Shh," says Gwendolyn. "Breathe."

"I'm not afraid of dead things," says Halah. "I've been through anatomy. I'm not afraid of ghosts."

"I never said you were."

The rasp of her breath slows, steadies. Tentatively, Gwendolyn moves to extricate herself. Halah lets her. In the wake of the near-catastrophe, her own adrenaline is flowing, making her heart pound.

"I have two daughters," Halah says. "One is in the astronautical service, like me. Do you have family?"

"Halah, please. Don't think about that."

"How?" Halah snaps. "Don't think about her, how? Four years ago, we should have been back. What happened to her? What happened to everyone? Was there a war, is my daughter dead? Are they all dead? If they aren't dead, why didn't anyone come back for us?"

And then, as if the panic and anger had been chisels against a dam inside her, the sadness bursts from her. Gwendolyn does not speak, does not move to touch the crying woman. Everyone must confront these moments; they must be resolved on one's own.

"I'm so sorry," Halah says at last. "This is a terrible place. The dark, and the creatures . . . I just hate it, I really hate it. I don't know what I'd do if you weren't here with me. Some diver."

"It's all right," says Gwendolyn. "We can leave the lamps on tonight, okay? Not everyone is a diver. You know if you need to travel, you can wait for a courier to come with you. You don't have to do this alone."

"Don't patronize me," says Halah, with a wry smile. "You know that I do."

"Well," says Gwendolyn. "Not tonight. Okay?"

The two lights shine out into the green.

Someone is tugging on her tether.

She starts from her reverie, sending glowing jellyfish scattering in the current. There is a voice, echoing through the water, calling "Gwendolyn, Gwendolyn." Reflexively she switches her light on, and in response, the voice makes a great whoop. It echoes all about her, booming and flat.

Five minutes later, Yiwei is there—her hair a little whiter than Gwendolyn remembers, and her suit painted with one of the garish, intricate floral designs that the biology folks have been playing with recently.

"Well, look at you!" Yiwei says—but she is actually looking at the jellyfish. "Man. Have you seen the marine ecologists lately? We'd better get them down here before all of these float completely out of tether range. Now I understand why didn't want to leave . . . did you bring a sample case? Where is my sample case?"

"Yiwei," says Gwendolyn. Her mouth feels strange. "What are you doing here?"

"Looking for you," Yiwei retorts. "We've been feeling vibrations in the axon since about four days ago. Usually once we feel it, a diver is there within two days, but nobody showed up. Marcus thought it was a big fish bumping the rail, but I thought I should check in case it was a diver after all. Lucky for you. How long have you been sitting there staring at pretty animals? Are you having some kind of trouble? Gwendolyn?"

Gwendolyn pulls up her clock. She must have slept, or been in a daze. Nearly two days have passed since she left the rail to follow the lights.

"Gwendolyn, your filter is clogged with tesselae," says Yiwei, coming behind her. Her hands are busy at the back of Gwendolyn's suit. "I want to get you back up to the anthro camp now, you're acting strange. I think you might be having a gas filtration problem. Don't worry, we'll find the fish again. Gwen? What's your message? Talk to me."

"Halah is dead," she says. "She went missing on a solo dive."

Yiwei pauses for a moment. "Shit," she says. "I know every loss diminishes us now, but . . . this one's personal, isn't it? Weren't you the one who trained her to dive? I'm really sorry, Gwendolyn."

Gwendolyn tries lifting her hand. Her movements feel sluggish; she's been still too long.

"I don't want you to set a marker," she says. "I don't want the marine ecologists to cut them up. They're for Halah. A memorial for the dead. For all of the dead. Just let them go."

"Gwen, I'm sorry, but if I did that, I'd be betraying my xenobiologist's soul," says Yiwei. "That's bioluminescence. That's an active chemical process. We might be able to date that. Do you know how big this is? Forget ecology, anthropology hasn't had a breakthrough like this in ten years."

Gwendolyn can't suppress her laugh, though her words come slow and difficult. "Come on. A breakthrough nobody will read about. A breakthrough that will never matter, because nobody will ever know."

At some point Yiwei must have come back around front of her, because now she's hooking her tether to the front of Gwendolyn's suit, her hands moving so fast that Gwendolyn can barely follow. Her vision is swimming. A problem with the filter? Was that what she said?

"I don't want to hear you talking like that," says Yiwei, her voice deliberately cheerful. "I'll know about it. You'll know about it. Maybe someday we'll go home, and everyone else will too. And if they never come . . . then I promise you, I will personally paint up my suit as a Rosetta stone for the next alien graverobbers to find. But now we have to go, okay? We don't have a lot of time. Swim."

The tug at her tether rouses her, and the two women kick up out of the depths.