The rover drew to a halt on the outskirts of the Thoth team’s sprawling camp, close to the foothills of the central peak. The dust here was churned up by rover tracks and flitter exhaust splashes, and semitransparent bubble-shelters were hemispheres of yellow, homely light, illuminating the darkened ice surface. There were drilling rigs, and several large pits dug into the ice.

Scholes helped Larionova out onto the surface. “I’ll take you to a shelter,” he said. “Or a flitter. Maybe you want to freshen up before—”

“Where’s Dixon?”

Scholes pointed to one of the rigs. “When I left, over there.”

“Then that’s where we’re going. Come on.”

Frank Dixon was the team leader. He met Larionova on the surface, and invited her into a small opaqued bubble-shelter nestling at the foot of the rig.

Scholes wandered off into the camp, in search of food.

The shelter contained a couple of chairs, a data desk, and a basic toilet. Dixon was a morose, burly American; when he took off his helmet there was a band of dirt at the base of his wide neck, and Larionova noticed a sharp, acrid stink from his suit. Dixon had evidently been out on the surface for long hours.

He pulled a hip flask from an environment suit pocket. “You want a drink?” he asked. “Scotch?”

“Sure.”

Dixon poured a measure for Larionova into the flask’s cap, and took a draught himself from the flask’s small mouth.

Larionova drank; the liquor burned her mouth and throat, but it immediately took an edge off her tiredness. “It’s good. But it needs ice.”

He smiled. “Ice we got. Actually, we have tried it; Mercury ice is good, as clean as you like. We’re not going to die of thirst out here, Irina.”

“Tell me what you’ve found, Frank.”

Dixon sat on the edge of the desk, his fat haunches bulging inside the leggings of his environment suit. “Trouble, Irina. We’ve found trouble.”

“I know that much.”

“I think we’re going to have to get off the planet. The System authorities — and the scientists and conservation groups — are going to climb all over us, if we try to mine here. I wanted to tell you about it, before—”

Larionova struggled to contain her irritation and tiredness. “That’s not a problem for Thoth,” she said. “Therefore it’s not a problem for me. We can tell Superet to bring in a water-ice asteroid from the Belt, for our supplies. You know that. Come on, Frank. Tell me why you’re wasting my time down here.”

Dixon took another long pull on his flask, and eyed her.

“There’s life here, Irina,” he said. “Life, inside this frozen ocean. Drink up; I’ll show you.”

The sample was in a case on the surface, beside a data desk. The thing in the case looked like a strip of multicolored meat: perhaps three feet long, crushed and obviously dead; shards of some transparent shell material were embedded in flesh that sparkled with ice crystals.

“We found this inside a two-thousand-yard-deep core,” Dixon said.

Larionova tried to imagine how this would have looked, intact and mobile. “This means nothing to me, Frank. I’m no biologist.”

He grunted, self-deprecating. “Nor me. Nor any of us. Who expected to find life, on Mercury?” Dixon tapped at the data desk with gloved fingers. “We used our desks’ medico-diagnostic facilities to come up with this reconstruction,” he said. “We call it a mercuric, Irina.”

A Virtual projected into space a foot above the desk’s surface; the image rotated, sleek and menacing.

The body was a thin cone, tapering to a tail from a wide, flat head. Three parabolic cups — eyes? — were embedded in the smooth “face,” symmetrically placed around a lipless mouth… No, not eyes, Larionova corrected herself. Maybe some kind of sonar sensor? That would explain the parabolic profile.

Mandibles, like pincers, protruded from the mouth. From the tail, three fins were splayed out around what looked like an anus. A transparent carapace surrounded the main body, like a cylindrical cloak; inside the carapace, rows of small, hairlike cilia lined the body, supple and vibratile.

There were regular markings, faintly visible, in the surface of the carapace.

“Is this accurate?”

“Who knows? It’s the best we can do. When we have your clearance, we can transmit our data to Earth, and let the experts get at it.”

“Lethe, Frank,” Larionova said. “This looks like a fish. It looks like it could swim. The streamlining, the tail—”

Dixon scratched the short hairs at the back of his neck and said nothing.

“But we’re on Mercury, damn it, not in Hawaii,” Larionova said.

Dixon pointed down, past the dusty floor. “Irina. It’s not all frozen. There are cavities down there, inside the Chao ice cap. According to our sonar probes—”

“Cavities?”

“Water. At the base of the crater, under a couple of miles of ice. Kept liquid by thermal vents, in crust-collapse scarps and ridges. Plenty of room for swimming… we speculate that our friend here swims on his back—” he tapped the desk surface, and the image swiveled “ — and the water passes down, between his body and this carapace, and he uses all those tiny hairs to filter out particles of food. The trunk seems to be lined with little mouths. See?” He flicked the image to another representation; the skin became transparent, and Larionova could see blocky reconstructions of internal organs. Dixon said, “There’s no true stomach, but there is what looks like a continuous digestive tube passing down the axis of the body, to the anus at the tail.”

Larionova noticed a threadlike structure wrapped around some of the organs, as well as around the axial digestive tract.

“Look,” Dixon said, pointing to one area. “Look at the surface structure of these lengths of tubing, here near the digestive tract.”

Larionova looked. The tubes, clustering around the digestive axis, had complex, rippled surfaces. “So?”

“You don’t get it, do you? It’s convoluted — like the surface of a brain. Irina, we think that stuff must be some equivalent of nervous tissue.”

Larionova frowned. Damn it, I wish I knew more biology. “What about this thread material, wrapped around the organs?”

Dixon sighed. “We don’t know, Irina. It doesn’t seem to fit with the rest of the structure, does it?” He pointed. “Follow the threads back. There’s a broader main body, just here. We think maybe this is some kind of parasite, which has infested the main organism. Like a tapeworm. It’s as if the threads are extended, vestigial limbs…”

Leaning closer, Larionova saw that tendrils from the wormthing had even infiltrated the brain-tubes. She shuddered; if this was a parasite, it was a particularly vile infestation. Maybe the parasite even modified the mercuric’s behavior, she wondered.

Dixon restored the solid-aspect Virtual.

Uneasily, Larionova pointed to the markings on the carapace. They were small triangles, clustered into elaborate patterns. “And what’s this stuff?”

Dixon hesitated. “I was afraid you might ask that.”

“Well?”

“…We think the markings are artificial, Irina. A deliberate tattoo, carved into the carapace, probably with the mandibles. Writing, maybe: those look like symbolic markings, with information content.”

“Lethe,” she said.

“I know. This fish was smart,” Dixon said.

The people, victorious, clustered around the warmth of their new Chimney. Recovering from their journey and from their battle-wounds, they cruised easily over the gardens of cilia-plants, and browsed on floating fragments of food.

It had been a great triumph. The Heads were dead, or driven off into the labyrinth of tunnels through the Ice. Strong-Flukes had even found the Heads’ principal nest here, under the silty floor of the cavern. With sharp stabs of her mandibles, Strong-Flukes had destroyed a dozen or more Head young.