Footprints, on the water ice shore: human prints, made by some heavy-treaded boot in a shallow nitrogen frost, quite clear.

“The warren’s bio systems are probably highly efficient recyclers, but nothing is perfect. They still need oxygen . . .”

Abil walked up to the prints. His own foot was larger, by a few sizes. Standing here, he saw that the prints led back, away from the oxygen lake, forming a path that snaked almost dead straight toward the central mountain. And when he looked the other way, beyond the lake, he saw more trails leading off toward the rim, the circular heap of corpses.

Those striations he thought he had seen on the ice, radiating inward from the rim wall, were actually ruts, he saw now, worn into water ice as hard as granite by the passage of countless feet, over countless years. All those journeys, he thought, shuddering, out to that great heaped-up pile of mummies. Year after year, generation after generation.

Dower hefted a weapon. “This is our way in. Form up.”

Abil stood at the head of his unit. Briefly he surveyed their faces. There were ten of them, all friends—even Denh. They would support him now to their deaths. But his stripes were only provisional, and he knew that if he fouled up they would chew jockey to replace him for the next drop, wherever and whenever it was.

That wasn’t going to happen. He grinned tightly. “Reds, forward.” They formed into two rows, with Abil at the head.

They trotted along the line of the path in the ice, keeping to either side of the rut, heading steadily toward the central mountain. The going turned out to be treacherous. Even away from the main paths the ice was worn slick by the passage of human feet. There were a few stumbles, and every so often there was a silent burst of vapor as somebody stepped into an oxygen puddle. Every time one of his unit took a pratfall Abil called a halt to run fresh equipment checks.

After about a mile Dower paused. The rut had led them to a crater in the ice, maybe ten yards across—no, Abil saw, the edges of this shallow pit were too sharp for that, its circular form too regular, and the base of the pit was smooth, gunmetal gray. Dower pressed her finger to the surface, and read Virtuals that danced before her Eyes. “Metal,” she said. She beckoned to Abil. “Corporal. Find a way in.”