Five metal streetlamps marked one channel for them, miraculously remaining upright after all these years, The conical algal encrustations around them were actually solidifying, turning them into cartoon desert islands. From the height of the poles left above the surface Greg guessed that the street must've been about one and a half metres below the hovercraft.

Further in, the mounds became more regular, the channels echoing the street pattern they covered. Sections of walls had survived here, triangular, cracked, and leaning at crooked angles. The brickwork was obscured by a viscid pebble-dash of gull droppings. An eerie desynchronised harmonic from the electric fans was bouncing back off them, amplifying their natural soft purr to a vociferous clattering reverberation.

Overhead, hundreds of gulls twisted in devious helices, calling shrilly, the high-decibel feedback from the entire flock a brazen fortissimo rolling across the ruins. Greg realised it was impossible to creep up on Wisbech.

They swept out of the mounds and into a suburb that was still standing; two-storey houses bordering a light industrial estate. The mud came halfway up the ground-floor windows.

There was no glass left in them. Second-storey windows were shattered, crystalline shark teeth sticking out of mouldering frames. Walls bulged, roofs sagged alarmingly, shedding tiles like autumn leaves. Gutters were wadded with grass and bindweed.

Moving on.

The Nene's old course was a serpentine semi-liquid desert, three hundred and fifty metres wide, flat and featureless. All the embankment buildings had been pulverised by the febrile floodwater, their debris sucked away by the inexorable vortices generated by the clash between currents of salt water and fresh water. Since then the eternal mud had oozed back, a great leveller.

Wisbech used to have a bustling port, the river lined by ugly warehouses and towering cranes. Greg had no way of telling where the iron titans had once stood.

Both hovercraft picked up speed on the flat. The heat pressed down, magnified by still, heavy air. Even the gulls abandoned the chase.

Greg received a pernicious impression of waiting depth. He was eager to reach the other side.

Their destination was becoming apparent straight ahead, on the other side of the old river course. The most prominent building there was. An old brick mill tower, slightly tapering, stained almost completely black with age.

Greg didn't understand how it could've possibly survived until they arrived at its base, riding noisily across the buckled corrugated roof of a petrol station which was elevated half a metre above the mud. The tower had been built on the summit of a raised stony mound. While chaos and ruin had boiled all around, it had remained aloof and untouched.

Tufts of tough Bermuda grass grew around its base; there was a good two metres of hard-packed earth between the bricks and the mud. The blades in front of the door were trampled down.