As it turned out, the arm had given her cause to think much about Alvin and where it had been, and in consequence where in God’s universe her enemy might have come from.

Late in the twentieth century, new oceans had begun turning up in the most unlikely places. Saturn’s little ice moon Enceladus, when Voyager II found her, appeared to be caught in a gravitational tug-of-war. On one side was Saturn. On the other side were Tethys—which sometimes swept past Enceladus at less than one-eighth the distance separating the Earth from the Moon—and Titan, which, though nearly twice as far from Enceladus as the Moon was from Earth, was nearly as massive as Mars. Enceladus’ crust was constantly straining and creaking, and as a result Enceladus, unlike comet Sargenti-Peterson and Ceres, had remained warm and wet inside for all the 4.6 billion years of its existence. Despite the fact that it could fit comfortably within the borders of New Mexico, and should have long ago been frozen to the core, it had become one of the most geologically active bodies in the solar system.

Because Enceladus had no atmosphere, water erupting through volcanic fissures in its mantle flashed to vapor, then froze instantly to snow. The first robot probes to park in Saturnian orbit found that Enceladus’ surface reflected as much sunlight as freshly fallen snow and saw trillions of twinkling ice crystals occupying the moon’s orbit, forming Saturn’s outermost ring. Seismic probes revealed a rocky core 155 kilometers wide, telling of a world within a world, a planet so small that a submarine could travel all the way around its 487-kilometer circumference in a single day.

But Enceladus’ mantle of ice was more than one hundred kilometers thick. Not even flights across interstellar distances totaling trillions of kilometers could provide an equivalent barrier. And there was nothing of value there, as far as anyone could tell. So the first Valkyries had set off for Jupiter and Alpha Centauri, and Ceres was in the process of being hollowed out when, near the end of man’s Earthly existence, funds were finally made available for the launch of Saturn’s first permanently crewed research center.

Nearer to home, and more thinly crusted, Jupiter’s second largest satellite, Europa, was locked in an even more powerful tug-of-war. Four ice volcanoes were erupting plumes of snow a hundred kilometers into space. Though slightly smaller than Earth’s moon, there were no mountains or crater rims on Europa. Its surface was as smooth as a billiard ball, and girded with ice-filled cracks, as if it were warm plastic constantly in motion, with every hill sinking under its own weight.

This ocean, unlike its Enceladan counterpart, was immediately accessible. And so it happened that one of the first Valkyries ever built, flying on a quantity of antihydrogen small enough to be contained in a beer can, was able to carry an entire space station and Alvin—and all the other equipment necessary for penetrating Europa’s thin crust—on a round-trip voyage lasting only three months. And, oh the sights that had awaited Hollis on the bottom of that hidden sea. She knew of a place where pillars of hardened lava were capped with creatures that looked and behaved like roses. They were colonies of living cells, barely more highly evolved than sponges. Lacking any trace of digestive systems, they lived by inhaling sulfides from basaltic rocks, manufacturing their own food and evolving, without benefit of solar energy, along a strange boundary between plants and animals. And she knew where there grew a mat of funguslike cells, all descended from the same seed. It thrived so prodigiously on volcanic sulfides that it had formed a carpet wider than Lake Superior, and Hollis guessed that, unless something even stranger turned up, it might actually be the largest single organism in the universe.