The blackness was absolute.

Watching over it was Louise Beno�t, twenty-eight, a statuesque postdoc from Montreal with a mane of thick brown hair stuffed, as required here, into a hair net. She kept her vigil in a cramped control room, buried two kilometers--"a mile an' a quarder," as she sometimes explained for American visitors in an accent that charmed them--beneath the Earth's surface.

The control room was next to the deck above the vast, unilluminated cavern housing the Sudbury Neutrino Observatory. Suspended in the center of that cavern was the world's largest acrylic sphere, twelve meters--"almost fordy feet"--across. The sphere was filled with eleven hundred tonnes of heavy water on loan from Atomic Energy of Canada Limited.

Enveloping that transparent globe was a geodesic array of stainless-steel struts, supporting 9,600 photomultiplier tubes, each cupped in a reflective parabola, each aimed in toward the sphere. All of this--the heavy water, the acrylic globe that contained it, and the enveloping geodesic shell--was housed in a ten-story-tall barrel-shaped cavern, excavated from the surrounding norite rock. And that gargantuan cavern was filled almost to the top with ultrapure regular water.

The two kilometers of Canadian shield overhead, Louise knew, protected the heavy water from cosmic rays. And the shell of regular water absorbed the natural background radiation from the small quantities of uranium and thorium in the surrounding rock, preventing that, too, from reaching the heavy water. Indeed, nothing could penetrate into the heavy water except neutrinos, those infinitesimal subatomic particles that were the subject of Louise's research. Trillions of neutrinos passed right through the Earth every second; in fact, a neutrino could travel through a block of lead a light-year thick with only a fifty-percent chance of hitting something.

Still, neutrinos poured out of the sun in such vast profusion that collisions did occasionally occur--and heavy water was an ideal target for such collisions. The hydrogen nuclei in heavy water each contain a proton--the normal constituent of a hydrogen nucleus--plus a neutron, as well. And when a neutrino did chance to hit a neutron, the neutron decayed, releasing a proton of its own, an electron, and a flash of light that could be detected by the photomultiplier tubes.

At first, Louise's dark, arching eyebrows did not rise when she heard the neutrino-detection alarm go ping; the alarm sounded briefly about a dozen times a day, and although it was normally the most exciting thing to happen down here, it still didn't merit looking up from her copy of Cosmopolitan.

But then the alarm sounded again, and yet again, and then it stayed on, a solid, unending electric bleep like a dying man's EKG.

Louise got up from her desk and walked over to the detector console. On top of it was a framed picture of Stephen Hawking--not signed, of course. Hawking had visited the Sudbury Neutrino Observatory for its grand opening a few years ago, in 1998. Louise tapped on the alarm's speaker, in case it was on the fritz, but the keening continued.

Paul Kiriyama, a scrawny grad student, dashed into the control room, arriving from elsewhere in the vast, underground facility. Paul was, Louise knew, usually quite flustered around her, but this time he wasn't at a loss for words. "What the heck's going on?" he asked. There was a grid of ninety-eight by ninety-eight LEDs on the detector panel, representing the 9,600 photomultiplier tubes; every one of them was illuminated.

"Maybe someone accidentally turned on the lights in the cavern," said Louise, sounding dubious even to herself.

The prolonged bleep finally stopped. Paul pressed a couple of buttons, activating five TV monitors slaved to five underwater cameras inside the observatory chamber. Their screens were perfectly black rectangles. "Well, if the lights were on," he said, "they're off now. I wonder what--"

"A supernova!" declared Louise, clapping her long-fingered hands together. "We should contact the Central Bureau for Astronomical Telegrams; establish our priority." Although SNO had been built to study neutrinos from the sun, it could detect them from anywhere in the universe.

Paul nodded and plunked himself down in front of a web browser, clicking on the bookmark for the Bureau's site. It was worth reporting the event, Louise knew, even if they weren't yet sure.

A new series of pings sounded from the detector panel. Louise looked at the LED board; several hundred lights were illuminated all over the grid. Strange, she thought. A supernova should register as a directional source...

"Maybe something's wrong with the equipment?" said Paul, clearly reaching the same conclusion. "Or maybe the connection to one of the photomultipliers is shorting out, and the others are picking up the arc."

The air split with a creaking, groaning sound, coming from next door--from the deck atop the giant detector chamber itself. "Perhaps we should turn on the chamber lights," said Louise. The groaning continued, a subterranean beast prowling in the dark.

"But what if it is a supernova?" said Paul. "The detector is useless with the lights on, and--"

Another loud cracking, like a hockey player making a slap shot. "Turn on the lights!"

Paul lifted the protective cover on the switch and pressed it. The images on the TV monitors flared then settled down, showing--

"Mon dieu," declared Louise.

"There's something inside the heavy-water tank!" said Paul. "But how could--?"

"Did you see that?" said Louise. "It's moving, and--good Lord, it's a man!"

The cracking and groaning sounds continued, and then--

They could see it on the monitors and hear it coming through the walls.

The giant acrylic sphere burst apart along several of the seams that held its component pieces together. "Tabernacle," Louise swore, realizing the heavy water must now be mixing with the regular H₂inside the barrel-shaped chamber. Her heart was jackhammering. For half a second, she didn't know whether to be more concerned about the destruction of the detector or about the man who was obviously drowning inside it.

"Come on!" said Paul, heading for the door leading to the deck above the observatory chamber. The cameras were slaved to VCRs; nothing would be missed.

"Un moment," said Louise. She dashed across the control room, grabbed a telephone handset, and pounded out an extension from the list taped to the wall.

The phone rang twice. "Dr. Montego?" said Louise, when the Jamaican-accented voice of the mine-site physician came on. "Louise Beno�t here, at SNO. We need you right away down at the neutrino observatory. There's a man drowning in the detector chamber."

"A man drowning?" said Montego. "But how could he possibly get in there?"

"We don't know. Hurry!"

"I'm on my way," said the doctor. Louise replaced the handset and ran toward the same blue door Paul had gone through earlier, which had since swung shut. She knew the signs on it by heart:

Louise grabbed the handle, pulling the door open, and hurried onto the wide expanse of the metal deck.

There was a trapdoor off to one side leading down to the actual detector chamber; the final construction worker had exited through it, and had sealed it shut behind him. To Louise's astonishment, the trapdoor was still sealed by forty separate bolts--of course, it was supposed to be sealed, but there was no way a man could have gotten inside except through that trapdoor...

The walls surrounding the deck were covered with dark green plastic sheeting to keep rock dust out. Dozens of conduits and polypropylene pipes hung from the ceiling, and steel girders sketched out the shape of the room. Computing equipment lined some walls; others had shelves. Paul was over by one of the latter, desperately rummaging around, presumably for pliers strong enough to crank the bolts free.

Metal screamed in anguish. Louise ran toward the trapdoor--not that there was anything she could do to unseal it with her bare hands. Her heart leapt; a sound like machine-gun fire erupted into the room as the restraining bolts shot into the air. The trapdoor burst open, slapping back on its hinges and hitting the deck with a reverberating clang. Louise had jumped out of the way, but a geyser of cold water leapt up through the opening, soaking her.

The very top of the detector chamber was filled with nitrogen gas, which Louise knew must be venting now. The water spout quickly subsided. She moved toward the opening in the deck and looked down, trying not to breathe. The interior was illuminated by the floodlights Paul had turned on, and the water was absolutely pure; Louise could see all the way to the bottom, thirty meters below.

She could just make out the giant curving sections of the acrylic sphere; the acrylic's index of refraction was almost identical to that of water, making it hard to see. The sections, separated from each other now, were anchored to the roof by synthetic-fiber cables; otherwise, they would have already sunk to the bottom of the surrounding geodesic shell. The trapdoor's opening gave only a limited perspective, and Louise couldn't yet see the drowning man.

"Merde!" The lights inside the chamber had gone off. "Paul!" Louise shouted. "What are you doing?"

Paul's voice--now coming from back in the control room--was barely audible above the air-conditioning equipment and the sloshing of the water in the huge cavern beneath Louise's feet. "If that man's still alive," he shouted, "he'll see the lights up on the deck through the trapdoor."

Louise nodded. The only thing the man would now be seeing was a single illuminated square, a meter on a side, in what, to him, would be a vast, dark ceiling.

A moment later, Paul returned to the deck. Louise looked at him, then back down at the open trapdoor. There was still no sign of the man. "One of us should go in," said Louise.

Paul's almond-shaped eyes went wide. "But ... the heavy water--"

"There's nothing else to do," said Louise. "How good a swimmer are you?"

Paul looked embarrassed; the last thing he ever wanted to do, Louise knew, was look bad in her eyes, but ... "Not very," he said, dropping his gaze.

It was already awkward enough down here with Paul mooning over her all the time, but Louise couldn't very well swim in her SNO-issue blue-nylon jumpsuit. Underneath, though, like almost everyone else who worked at SNO, she only had on her underwear; the temperature was a tropical 40.6 Celsius this far beneath Earth's surface. She yanked off her shoes, then pulled on the zipper that ran down the front of the jumpsuit; thank God she'd worn a bra today--although she wished now that it hadn't been as lacy.

"Turn the lights back on down there," said Louise. To his credit, Paul didn't tarry. Before he'd returned, Louise had slipped through the trapdoor into the cold water; the water was chilled to ten degrees Celsius to discourage biological growth and to reduce the spontaneous noise rate of the photomultiplier tubes.

She felt a rush of panic, a sudden feeling of being a long way up with nothing supporting her; the bottom was far, far below. She was treading water, her head and shoulders sticking up through the open trapdoor into the air, waiting for her panic to subside. When it did, she took three deep breaths, closed her mouth tight, and dived beneath the surface.

Louise could see clearly, and her eyes didn't sting at all. She looked around, trying to spot the man, but there were so many pieces of acrylic, and--

There he was.

He had indeed floated up, and there was a small gap--maybe fifteen centimeters--between the top of the water and the deck above. Normally it was filled with ultrapure nitrogen. The poor guy must be dead; three breaths of that would be fatal. A sad irony: he probably fought his way to the surface, thinking he could find air, only to be killed by the gas he inhaled there. Breathable air from the open trapdoor must now be mixing with the nitrogen, but presumably it was too late to help him.

Louise pushed her own head and shoulders up through the trapdoor again. She could see Paul, desperately waiting for her to say something--anything. But there was no time for that. She gulped more air, filling her lungs as much as she could, then dived under. There wasn't enough room for her to keep her nose above water without constantly banging her head into the metal roof as she swam. The man was about ten meters away. Louise kicked her feet, covering the distance as quickly as she could, and--

A cloud in the water. Something dark.

Mon dieu!

It was blood.