She looked dubious. “I wouldn’t like to try. It’s only clear for an hour. Something to do with another flood which moves in this way from the Atlantic.”

A mile away the great, jagged rock of St. Pierre lifted out of the sea. The castle was perched on the ultimate edge of the cliffs, its strange, pointed Gothic towers in sharp relief against the blue sky. The sea creamed over rocks two hundred feet below.

“What do you think of it?” she said.

“It must have cost a fortune to build even on the golden tide of Victorian prosperity.” He shaded his eyes and frowned. “I can’t see a jetty.”

“It’s under the island. If you look carefully beyond the last line of rocks you’ll see the entrance in the cliffs. At high water there’s only ten- or twelve-foot clearance.”

“Is the water very deep in there?” he said casually.

She nodded. “Even at low water there’s a good ten fathoms. There’s a fault in the sea-bed which splits the reef along the centre. It runs right under St. Pierre.”

“Would that be your famous Middle Passage?”

“That’s right, and it’s well worth seeing.”

She clamped her rubber mouthpiece between her teeth, pulled down the mask and eased herself back into the water. Visibility was still good and Mallory could see the great boulders of the reef four or five fathoms beneath, and then quite suddenly Anne tilted her scooter over a shelf of granite and went down into space.

They moved through a misty tunnel of rock, sunlight slanting through fissures and cracks in the roof in wavering bands of light. In places the passage was reminiscent of a cathedral nave, the rock arched up on either side to support the roof, and then the dimness brightened and they moved into a section which was open to the sea.

Anne was twenty or thirty feet in front and she paused, waiting for him. When he approached she jack-knifed. Mallory went after her, the scarlet nose of his scooter cleaving the water, fish crowding to either side. At ten fathoms he moved into a mysterious green dusk with visibility considerably reduced.

Beneath him she had paused, hovering over a ledge. When he joined her he saw, to his astonishment, a three-ton Bedford truck wedged on its side in a large fissure. The canvas tilt had long since disappeared, but when he moved in close he saw painted on the side the white star which all Allied vehicles had carried on D-Day and after.

They moved away again and a moment later the outline of a ship’s stern loomed out of the gloom Every rail, every line, was festooned with strange submarine growths and he followed the curving side to where a ragged torpedo hole gaped darkly at him. Beneath, tilted into a crevasse, was a Churchill tank, beyond it, the shapes of trucks, a solitary field-gun’s barrel slanting towards the surface.

Mallory followed Anne across the deck to the wheelhouse. The open door swung gently in the current, the deck around it smashed and broken as if by some internal explosion. The wheel was still intact and also the compass in its mounting, encrusted with scales. When Mallory moved inside he had a strange sensation that someone should be there, that something was missing. A bad end for a good ship, he thought, and moved out again. She tapped him on the shoulder, and together they rose towards the luminosity that was the surface.

They hauled themselves on to the flat top of a large rock, dry in the sun, and Anne pulled up her mask and breathed deeply several times.

“What’s the story?” Mallory said.

She shrugged. “One of the D-Day armada that never made it. She was torpedoed near Guernsey. When her engines stopped the tide carried her straight in across the reef. Apparently, the crew got away earlier in the lifeboats.”

“Where did you get the story?”

“From Owen Morgan. There are plenty of wrecks in these waters and Owen knows them all – and their histories. Something of a hobby with him.”

“Interesting,” Mallory said. “I’d like to take another look. Feel up to it?”

“I don’t think so. I’ll wait for you here. Don’t stay too long. When the tide starts turning there’s quite an undercurrent through the passage.”

He was aware of it almost at once like an invisible hand pushing him to one side as he went down over the edge of the reef. The pressure of the water clawed at his mask as the scooter pulled him down and he swerved as a steel mast pierced the gloom.

He hovered over the tilting deck, considering his next move. The sight of the black, gaping entrance to a companionway decided him He moved inside, switching on the spot which was mounted on top of the scooter.

He moved along the angled corridor and opened the first door he came to. It fell inwards slowly, the room beyond it dark and he was aware of a strange, irrational fear. He pushed forward boldly, and the light, spreading through the water, showed him a table bolted to the floor, a bunk against one wall and bottles and assorted debris floating against the ceiling.

He swam out and moved further along the corridor to where it disintegrated into a twisted mass of metal, electric wires draped from the roof and, most poignant sight of all, the broken remnants of a human skeleton crushed beneath a girder.

He moved back along the corridor quickly and, the moment he emerged from the companionway, struck up towards the reef. At twenty feet he paused to decompress for several minutes, aware of the current tugging at his body. He surfaced a few yards from the rock and found Anne Grant waist-deep on the edge of the reef, adjusting her equipment.

“We’ll have to get moving,” she shouted, as he approached and pushed up his mask. “It must be later than I thought. I can feel the tide moving already.”

“Is that bad?” he said.

She nodded. “Even the aquamobiles aren’t going to do us much good with a five-knot current flowing the other way.”

She moved off at once and he went after her. Behind them the entire length of the reef was surging into breakers and he could feel the relentless pressure of the current. He started to flutter-kick with all his strength, and gradually the point grew nearer. Anne turned, gave him a quick wave and they went down.

He could see the weeds on the sea-bed beneath him leaning over on one side, pointing back towards the reef, and the pressure was now a solid wall that he was trying to break through. He kicked again, was dimly aware of the black rocks passing beneath him and then they were round into calm water and his aquamobile seemed to leap forward with a surge of power.

He surfaced and saw Anne at once, over to the right and some distance in front of him. He raised a hand, urging her on, and followed. When he rounded the final point of rock she was perhaps fifty yards in front of him and moving strongly towards Foxhunter.

A speedboat was moored beside the ladder, the sunlight gleaming on its scarlet trim, and someone sat in a canvas chair next to General Grant, a tall, distinguished-looking man in dark glasses and linen jacket who stood up and moved to the rail, shading his eyes as Anne approached.

She reached the ladder and he moved to give her a hand. When she climbed up on deck Mallory was still twenty or thirty yards away and he reduced speed.

As he came in under the counter of the speedboat the man who was sitting at the wheel turned to look down at him. He was a large, dangerous-looking individual with a hard face, a jagged scar bisecting the right cheek. Mallory recognised him at once from the photograph he had been shown at his briefing.

He pushed up his mask. “Hello there.”

Jacaud looked down at him calmly, nodded, then turned away. Mallory pulled himself to the bottom of the ladder where Raoul Guyon was already waiting, a hand outstretched for the aquamobile.

Mallory went over the rail, squatted on deck and took off his aqualung. Anne Grant was standing a yard or two away, an attractive figure in her yellow diving suit as she talked to the man in the linen jacket.