Screeching like a dispossessed banshee, a large hammer-nosed bat soared straight out of one of the narrow inlets off the creek and swerved straight toward the cutter. Its sonar confused by the labyrinth of giant webs spun across the inlet by the colonies of wolf spiders, it missed the wire hood above Kerans' head by only a few feet, and then sailed away along the line of submerged office blocks, gliding in and out of the huge sail-like fronds of the fern trees sprouting from their roofs. Suddenly, as it passed one of the projecting cornices, a motionless stone-headed creature snapped out and plucked the bat from the air. There was a brief piercing squawk and Kerans caught a glimpse of the crushed wings clamped in the lizard's jaws. Then the reptile shrank back invisibly among the foliage.

All the way down the creek, perched in the windows of the office blocks and department stores, the iguanas watched them go past, their hard frozen heads jerking stiffly. They launched themselves into the wake of the cutter, snapping at the insects dislodged from the air-weed and rotting logs, then swam through the windows and clambered up the staircases to their former vantage points, piled three deep across each other. Without the reptiles, the lagoons and the creeks of office blocks half-submerged in the immense heat would have had a strange dream-like beauty, but the iguanas and basilisks brought the fantasy down to earth. As their seats in the one-time boardrooms indicated, the reptiles had taken over the city. Once again they were the dominant form of life.

Looking up at the ancient impassive faces, Kerans could understand the curious fear they roused, rekindling archaic memories of the terrifying jungles of the Paleocene, when the reptiles had gone down before the emergent mammals, and sense the implacable hatred one zoological class feels towards another that usurps it.

At the end of the creek they entered the next lagoon, a wide circle of dark green water almost half a mile in diameter. A lane of red plastic buoys marked a channel towards an opening on the far side. The cutter had a draught of little more than a foot, and as they moved along through the flat water, the sun slanting down behind them opening up the submerged depths, they could see the clear outlines of five- and six-storey buildings looming like giant ghosts, here and there a moss-covered roof breaking the surface as the swell rolled past it.

Sixty feet below the cutter a straight grey promenade stretched away between the buildings, the remains of some former thoroughfare, the rusting humped shells of cars still standing by the curb. Many of the lagoons in the centre of the city were surrounded by an intact ring of buildings, and consequently little silt had entered them. Free of vegetation, apart from a few drifting clumps of Sargasso weed, the streets and shops had been preserved almost intact, like a reflection in a lake that has somehow lost its original.

The bulk of the city had long since vanished, and only the steel-supported buildings of the central commercial and financial areas had survived the encroaching flood waters. The brick houses and single-storey factories of the suburbs had disappeared completely below the drifting tides of silt. Where these broke surface giant forests reared up into the burning dull-green sky, smothering the former wheatfields of temperate Europe and North America. Impenetrable Matto Grossos sometimes three hundred feet high, they were a nightmare world of competing organic forms returning rapidly to their Paleozoic past, and the only avenues of transit for the United Nations military units were through the lagoon systems that had superimposed themselves on the former cities. But even these were now being clogged with silt and then submerged.

Kerans could remember the unending succession of green twilights that had settled behind them as he and Riggs moved slowly northward across Europe, leaving one city after another, the miasmic vegetation swamping the narrow canals and crowding from rooftop to rooftop.

Now they were to abandon yet another city. Despite the massive construction of the main commercial buildings, it consisted of little more than three principal lagoons, surrounded by a nexus of small lakes fifty yards in diameter and a network of narrow creeks and inlets which wound off, roughly following the original street-plan of the city, into the outlying jungle. Here and there they vanished altogether or expanded into the steaming sheets of open water that were the residues of the former oceans. In turn these gave way to the archipelagoes that coalesced to form the solid jungles of the southern massif.

The military base set up by Riggs and his platoon, which harboured the biological testing station, was in the most southerly of the three lagoons, sheltered by a number of the tallest buildings of the city, thirty-storey blocks in what had once been the down-town financial sector.

As they crossed the lagoon the yellow-striped drum of the floating base was on its sun-ward side, almost obscured in the reflected light, the rotating blades of the helicopter on its roof throwing brilliant lances across the smaller white-painted hull of the biological testing water at them. Two hundred yards down shore was the smaller white-pointed hull of the biological testing station, moored against a broad hump-backed building which had formerly been a concert hall.

Kerans gazed up at the rectangular cliffs, enough of the windows intact to remind him of the illustrations of sun-dazzled promenades at Nice, Rio and Miami he had read about as a child in the encyclopaedias at Camp Byrd. Curiously, though, despite the potent magic of the lagoon worlds and the drowned cities, he had never felt any interest in their contents, and never bothered to identify which of the cities he was stationed in.

Dr. Bodkin, twenty-five years his senior, had actually lived in several of them, both in Europe and America, and spent most of his spare time punting around the remoter water-ways, searching out former libraries and museums. Not that they contained anything other than his memories.

Perhaps it was this absence of personal memories that made Kerans indifferent to the spectacle of these sinking civilisations. He had been born and brought up entirely within what had once been known as the Arctic Circle -now a sub-tropical zone with an annual mean temperature of eighty-five degrees-and had come southward only on joining one of the ecological surveys in his early 30's. The vast swamps and jungles had been a fabulous laboratory, the submerged cities little more than elaborate pedestals.

Apart from a few older men such as Bodkin there was no-one who remembered living in them-and even during Bodkin's childhood the cities had been beleagured citadels, hemmed in by enormous dykes and disintegrated by panic and despair, reluctant Venices to their marriage with the sea. Their charm and beauty lay precisely in their emptiness, in the strange junction of two extremes of nature, like a discarded crown overgrown by wild orchids.

The succession of gigantic geophysical upheavals which had transformed the Earth's climate had made their first impact some sixty or seventy years earlier. A series of violent and prolonged solar storms lasting several years caused by a sudden instability in the Sun had enlarged the Van Allen belts and diminished the Earth's gravitational hold upon the outer layers of the ionosphere. As these vanished into space, depleting the Earth's barrier against the full impact of solar radiation, temperatures began to climb steadily, the heated atmosphere expanding outwards into the ionosphere where the cycle was completed.

All over the world, mean temperatures rose by a few degrees each year. The majority of tropical areas rapidly became uninhabitable, entire populations migrating north or south from temperatures of a hundred and thirty and a hundred and forty degrees. Once-temperate areas became tropical, Europe and North America sweltering under continuous heat waves, temperatures rarely falling below a hundred degrees. Under the direction of the United Nations, the colonisation began of the Antarctic plateau and of the northern borders of the Canadian and Russian continents.

Over this initial period of twenty years a gradual adjustment of life took place to meet the altered climate. A slackening of the previous tempo was inevitable, and there was little spare energy available to cut back the encroaching jungles of the equatorial region. Not only was the growth of all plant forms accelerated, but the higher levels of radioactivity increased the rate at which mutations occurred. The first freak botanical forms appeared, recalling the giant tree-ferns of the Carboniferous period, and there was a drastic upsurge of all lower plant and animal forms.

The arrival of these distant forbears was overlayed by the second major geophysical upheaval. The continued heating of the atmosphere had begun to melt the polar ice-caps. The entrained ice-seas of the Antarctic plateau broke and dissolved, tens of thousands of glaciers around the Arctic Circle, from Greenland and Northern Europe, Russia and North America, poured themselves into the sea, millions of acres of permafrost liquefied into gigantic rivers.

Here again the rise of global water levels would have been little more than a few feet, but the huge discharging channels carried with them billions of tons of top-soil. Massive deltas formed at their mouths, extending the continental coastlines and damming up the oceans. Their effective spread shrank from two-thirds of the world's area to only slightly more than half.

Driving the submerged silt before them, the new seas completely altered the shape and contours of the continents. The Mediterranean contracted into a system of inland lakes, the British Isles was linked again with northern France. The Middle West of the United States, filled by the Mississippi as it drained the Rocky Mountains, became an enormous gulf opening into the Hudson Bay, while the Caribbean Sea was transformed into a desert of silt and salt flats. Europe became a system of giant lagoons, centred on the principal low-lying cities, inundated by the silt carried southwards by the expanding rivers.

During the next thirty years the pole-ward migration of populations continued. A few fortified cities defied the rising waterlevels and the encroaching jungles, building elaborate sea-walls around their perimeters, but one by one these were breached. Only within the former Arctic and Antarctic Circles was life tolerable. The oblique incidence of the Sun's rays provided a shield against the more powerful radiation. Cities on higher ground in mountainous areas nearer the Equator had been abandoned despite their cooler temperatures because of the diminished atmospheric protection.