Armies of biologists could have spent lifetimes studying one small oasis. Unlike the Palaeozoic terrestrial seas, the Europan abyss was not a stable environment, so evolution had progressed with astonishing speed, producing multitudes of fantastic forms. And all were under the same 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. All across the Europan sea-bed was 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. Some had left as their only memorial huge, empty shells like convoluted trumpets, larger than a man. And there were clams of many shapes – bivalves, and even trivalves, as well as spiral stone patterns, many metres across – exactly like the beautiful ammonites that disappeared so mysteriously from Earth's oceans at the end of the Cretaceous Period.

Among the greatest wonders of the Europan abyss were rivers of incandescent lava, pouring from the calderas of submarine volcanoes. The pressure at these depths was so great that the water in contact with the red-hot magma could not flash into steam, so the two liquids co-existed in an uneasy truce.

There, on another world and with alien actors, something like the story of Egypt had been played out 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 a few kilometres wide, species after species had evolved and flourished and passed away. And some had left permanent monuments.

Often, they were not easy to distinguish from the natural formations around the thermal vents, and even when they were clearly not due to pure chemistry, one would be hard put to decide whether they were the product of instinct or intelligence. On Earth, the termites reared condominiums almost as impressive as any found in the single vast ocean that enveloped this frozen world.

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 their oases were as isolated from one another as the planets themselves, The creatures who basked in the glow of the lava rivers, 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.

Yet even the space between the oases was not altogether empty of life; there were hardier creatures who had dared its rigours. Some were the Europan analogues of fish – streamlined torpedoes, propelled by vertical tails, steered by fins along their bodies. The resemblance to the most successful dwellers in Earth's oceans was inevitable; given the same engineering problems, evolution must produce very similar answers. Witness the dolphin and the shark – superficially almost identical, yet from far distant branches of the tree of life.

There was, however, one very obvious difference between the fish of the Europan seas and those in terrestrial oceans; they had no gills, for there was hardly a trace of oxygen to be extracted from the waters in which they swam. Like the creatures around Earth's own geothermal vents, their metabolism was based on sulphur compounds, present in abundance in this volcanic environment.

And very few had eyes. Apart from the flickering glow of lava outpourings, and occasional bursts of bioluminescence from creatures seeking mates, or hunters questing prey, it was a lightless world.

It was also a doomed one. Not only were its energy sources sporadic and constantly shifting, but the tidal forces that drove them were steadily weakening. Even if they developed true intelligence, the Europans were trapped between fire and ice.

Barring a miracle, they would perish with the final freezing of their little world.

Lucifer had wrought that miracle.

In the final moments, as he came in over the coast at a sedate hundred kilometres an hour, Poole wondered if there might be some last-minute intervention. But nothing untoward happened, even when he moved slowly along the black, forbidding face of the Great Wall.

It was the inevitable name for the Europa Monolith as, unlike its little brothers on Earth and Moon, it was lying horizontally, and was more than twenty kilometres long. Although it was literally billions of times greater in volume than TMA ZERO and TMA ONE, its proportions were exactly the same – that intriguing ratio 1:4:9, inspirer of so much numerological nonsense over the centuries.

As the vertical face was almost ten kilometres high, one plausible theory maintained that among its other functions the Great Wall served as a wind-break, protecting Tsienville from the ferocious gales that occasionally roared in from the Sea of Galilee. They were much less frequent now that the climate had stabilized, but a thousand years earlier they would have been a severe discouragement to any life-forms emerging from the ocean.

Though he had fully intended to do so, Poole had never found time to visit the Tycho Monolith – still Top Secret when he had left for Jupiter – and Earth's gravity made its twin at Olduvai inaccessible to him. But he had seen their images so often that they were much more familiar than the proverbial back of the hand (and how many people, he had often wondered, would recognize the backs of their hands?). Apart from the enormous difference in scale, there was absolutely no way of distinguishing the Great Wall from TMA ONE and TMA ZERO – or, for that matter, the 'Big Brother' Monolith that Discovery and the Leonov had encountered orbiting Jupiter.

According to some theories, perhaps crazy enough to be true, there was only one archetypal Monolith, and all the others – whatever their size – were merely projections or images of it. Poole recalled these ideas when he noticed the spotless, unsullied smoothness of the Great Wall's towering ebon face. Surely, after so many centuries in such a hostile environment, it should have collected a few patches of grime! Yet it looked as immaculate as if an army of window-cleaners had just polished every square centimetre.

Then he recalled that although everyone who had ever come to view TMA ONE and TMA ZERO felt an irresistible urge to touch their apparently pristine surfaces, no one had ever succeeded. Fingers – diamond drills – laser knives – all skittered across the Monoliths as if they were coated by an impenetrable film. Or as if – and this was another popular theory – they were not quite in this universe, but somehow separated from it by an utterly impassable fraction of a millimetre.

He made one complete, leisurely circuit of the Great Wall, which remained totally indifferent to his progress. Then he brought the shuttle – still on manual, in case Ganymede Control made any further attempts to 'rescue' him – to the outer limits of Tsienville, and hovered there looking for the best place to land.

The scene through Falcon's small panoramic window was wholly familiar to him; he had examined it so often in Ganymede recordings, never imagining that one day he would be observing it in reality. The Europs, it seemed, had no idea of town planning; hundreds of hemispherical structures were scattered apparently at random over an area about a kilometre across. Some were so small that even human children would feel cramped in them; though others were big enough to hold a large family, none was more than five metres high.