In a few revolutions, he saw more of Ganymede than all the space probes ever sent from Earth, and filed away the knowledge for future use. One day it would be important; he was sure of that, though he did not know why – any more than he understood the impulse that was now driving him so purposefully from world to world.

As, presently, it brought him to Europa. Though he was still largely a passive spectator, he was aware now of a rising interest, a focusing of attention – a concentration of will. Even if he was a puppet in the hands of an unseen and uncommunicative master, some of the thoughts of that controlling influence leaked – or were allowed to leak – into his own mind.

The smooth, intricately patterned globe now, rushing toward him bore little resemblance either to Ganymede or Callisto. It looked organic; the network of lines branching and intersecting over its entire surface was uncannily like a world-spanning system of veins and arteries.

The endless ice fields of a frigid waste, far colder than the Antarctic, stretched beneath him. Then, with brief surprise, he saw that he was passing over the wreckage of a spaceship. He recognized it instantly as the ill-fated Tsien, featured in so many of the video newscasts he had analysed. Not now – not now – there would be ample opportunity later.

Then he was through the ice, and into a world as unknown to his controllers as to himself.

It was an ocean world, its hidden waters protected from the vacuum of space by a crust of ice. In most places the ice was kilometres thick, but there were lines of weakness where it had cracked open and torn apart. Then there had been a brief battle between two implacably hostile elements that came into direct contact on no other world in the Solar System. The war between Sea and Space always ended in the same stalemate; the exposed water simultaneously boiled and froze, repairing the armour of ice.

The seas of Europa would have frozen completely solid long ago without the influence of nearby Jupiter. Its gravity continually kneaded the core of the little world; the forces that convulsed Io were working there, though with much less ferocity. As he skimmed across the face of the deep, he saw everywhere the evidence of that tug-of-war between planet and satellite.

And he both heard and felt it, in the continual roar and thunder of submarine earthquakes, the hiss of escaping gases from the interior, the infrasonic pressure waves of avalanches sweeping over the abyssal plains. By comparison with the tumultuous ocean that covered Europa, even the noisy seas of Earth were silent.

He had not lost his sense of wonder, and the first oasis filled him with delighted surprise. It extended for almost a kilometre around a tangled mass of pipes and chimneys deposited by mineral brines gushing from the interior. Out of that natural parody of a Gothic castle, black, scalding liquids pulsed in a slow rhythm, as if driven by the beating of some mighty heart. And, like blood, they were the authentic sign of life itself.

The boiling fluids drove back the deadly cold leaking down from above, and formed an island of warmth on the seabed. Equally important, they brought from Europa's interior all the chemicals of life. There, in an environment where none had expected it, were energy and food, in abundance.

Yet it should have been expected; he remembered that, only a lifetime ago, such fertile oases had been discovered in the deep oceans of Earth. Here they were present on an immensely larger scale, and in far greater variety.

In the tropical zone close to the contorted walls of the 'castle' were delicate, spidery structures that seemed to be the analogy of plants, though almost all were capable of movement. Crawling among these were bizarre slugs and worms, some feeding on the plants, others obtaining their food directly from the mineral-laden waters around them. At greater distances from the source of heat – the submarine fire around which all the creatures warmed themselves – were sturdier, more robust organisms, not unlike crabs or spiders.

Armies of biologists could have spent lifetimes studying that one small oasis. Unlike the Palaeozoic terrestrial seas, it was not a stable environment, so evolution had progressed swiftly here, producing multitudes of fantastic forms. And they were all under indefinite stay of execution; sooner or later, each fountain of life would weaken and die, as the forces that powered it moved their focus elsewhere.

Again and again, in his wanderings across the Europan seabed, he encountered the evidence of such tragedies. Countless circular areas were littered with the skeletons and mineral-encrusted remains of dead creatures, where entire chapters of evolution had been deleted from the book of life.

He saw huge, empty shells formed like convoluted trumpets as large as a man. There were clams of many shapes – bivalves, and even trivalves. And there were spiral stone patterns, many metres across, which seemed an exact analogy of the beautiful ammonites that disappeared so mysteriously from Earth's oceans at the end of the Cretaceous Period.

Searching, seeking, he moved back and forth over the face of the abyss. Perhaps the greatest of all the wonders he met was a river of incandescent lava, flowing for a hundred kilometres along a sunken valley. The pressure at that depth was so great that the water in contact with the red-hot magma could not flash into steam, and the two liquids coexisted in an uneasy truce.

There, on another world and with alien actors, something like the story of Egypt had been played long before the coming of man. As the Nile had brought life to a narrow ribbon of desert, so this river of warmth had vivified the Europan deep. Along its banks, in a band never more than two kilometres wide, species after species had evolved and flourished and passed away. And at least one had left a monument behind it.

At first, he thought that it was merely another of the encrustations of mineral salts that surrounded almost all the thermal vents. However, as he came closer, he saw that it was not a natural formation, but a structure created by intelligence. Or perhaps by instinct; on Earth, the termites reared castles that were almost equally imposing, and the web of a spider was more exquisitely designed.

The creatures that had lived there must have been quite small, for the single entrance was only half a metre wide. That entrance – a thick-walled tunnel, made by heaping rocks on top of each other – gave a clue to the builders' intentions. They had reared a fortress, there in the flickering glow not far from the banks of their molten Nile. And then they had vanished.

They could not have left more than a few centuries before. The walls of the fortress, built from irregularly shaped rocks that must have been collected with great labour, were covered with only a thin crust of mineral deposits. One piece of evidence suggested why the stronghold had been abandoned. Part of the roof had fallen in, perhaps owing to the continual earthquakes; and in an underwater environment, a fort without a roof was wide open to an enemy.

He encountered no other sign of intelligence along the river of lava. Once, however, he saw something uncannily like a crawling man – except that it had no eyes and no nostrils, only a huge, toothless mouth that gulped continuously, absorbing nourishment from the liquid medium around it.

Along the narrow band of fertility in the deserts of the deep, whole cultures and even civilizations might have risen and fallen, armies might have marched (or swum) under the command of Europan Tamberlanes or Napoleons. And the rest of their world would never have known, for all those oases of warmth were as isolated from one another as the planets themselves. The creatures who basked in the glow of the lava river, and fed around the hot vents, could not cross the hostile wilderness between their lonely islands, If they had ever produced historians and philosophers, each culture would have been convinced that it was alone in the Universe.