the wind was freshening, lifting the waves into white-caps, and as the dinghy rounded the point water slopped over the gunwale. Guyon carefully eased his weight into the centre and started to bale. He wore a heavy sweater and reefer jacket against the cold. A pair of night-glasses hung around his neck and one of the aquamobiles lay in the prow behind him.

Mallory sat in the stern wearing a black rubber diving suit, the heavy aqualung already strapped into place on his back. As a cross-current started to turn the dinghy in towards the cliffs he opened the throttle on the outboard motor to compensate and glanced at the luminous dial of his watch.

It was 11.45 and there was very little cloud, the sky brilliant with stars, and the moonlight danced across the waves, leaving a trail of silver behind it. The dinghy lifted high on a large swell and swung in towards the great finger of rock which marked the western tip of the island. Mallory opened the throttle again. For a moment the dinghy seemed to stand still and then it forged ahead.

They rounded the point, fighting the cross-currents, Guyon cursing steadily as water slopped over the sides, and then they were sweeping into calmer water. Beyond, St. Pierre and the Gothic towers of the castle were dark against the sky.

Mallory throttled down again and the dinghy coasted on, the sound of her motor a murmur on the wind. The great reef running between the two islands was deceptively innocent in the moonlight. Waves rolling in from the sea splashed lazily across the rocks, now and then a curtain of white spray lifting into the night like silver lace.

He took the dinghy into the calm waters of the Middle Passage until they reached the first point where the roof closed in and water boiled across great jagged black teeth. He cut the motor and the dinghy slowed and ground gently against a sloping, weed-covered shoulder of rock. Guyon hooked the painter into a crevasse and looked towards St. Pierre through the night-glasses.

“About a quarter of a mile. A long swim.”

“Not with the aquamobile,” Mallory said.

Guyon got it over the side, the dinghy heeling dangerously. “Rather you than me. The water’s like ice. How long will you be?”

Mallory shrugged. “No more than half an hour. I’ve no intention of hanging around at the other end.”

He fitted the rubber mouthpiece between his teeth and adjusted his air supply, touched the knife briefly at his belt and clambered awkwardly over the side on to the reef. He waded into the water, swam to the other side of the dinghy and reached for the aquamobile. Guyon smiled once and Mallory nodded and sank beneath the surface.

Moonlight filtered down through the water, probing into the depths. When he passed beneath the surface of the reef and came into the Middle Passage he entered a darker, more sinister, world.

He switched on the powerful spot mounted on top of the scooter and the shaft of light pierced through the darkness in front of him, splaying against the rocks that arched above his head.

He tilted the nose of the aquamobile and- went down gently, leveling out at twenty feet. Although his top speed was no more than three knots, he seemed to rush at a terrifying speed into the wall of grey mist that was the edge of his visibility. The great, arched nave of the reef stretched into infinity before him, the water breaking against his mask.

And then he was through and moving into a strange, unreal landscape of jumbled rocks and pale forests of seaweed waving languidly in the diffused moonlight. He surfaced and looked up at the cliffs of St. Pierre, the pointed towers of the castle dark against the sky.

The moonlight splashed across the face of the cliffs, picking out the dark mouth of the cave. It was now high water and there was no more than a ten- or twelve-foot clearance. Mallory turned the nose of the aquamobile down and leveled out at forty feet. He switched off the.spot and moved into a grey phosphorescent mist.

The great fault in the sea-bed dropped beneath him. At least ten fathoms, Anne had said, slicing into the heart of the island. The mist seemed to swing to one side like a curtain, revealing the entrance to the cave, a good sixty feet across as it widened on its way down.

He drifted in, grey-green walls moving past on either side. The water lightened, the grey merging into aquamarine as artificial light seeped down from the surface. He moved in close to the wall and went forward cautiously.

He stopped abruptly, switching off the aquamobile. From this point on the rough wall of the cave merged into the jetty, great square blocks of masonry like the foundations of some ancient fort descending into the depths. He started up cautiously and immediately the grey-black underbelly of the submarine appeared from the mist.

He had found what he was looking for and to stay any longer was to invite trouble. He turned and flutter-kicked towards the entrance. The light dimmed and he was aware of the current tugging at him.

He swam out into that strange, grey, phosphorescent world and paused to switch on the aquamobile. In that same moment it was torn from his grasp with a metallic clang and a shock-wave, spreading through the water, burst around him.

He turned and saw the frogman suspended in the water about twenty feet away, a weird sea-creature, full of menace, the moonlight glinting on his visor as he reloaded his spear-gun.

Mallory drove forward, pulling the heavy knife from its sheath. When he was perhaps ten feet away the gun exploded again in a shower of silver bubbles. He swung desperately to one side. The spear hurtled past and he moved in fast, his knife cleaving through rubber and flesh.

The man’s body bucked agonisingly, blood rising in a dark cloud as Mallory pulled out the knife and snatched at the air-pipe. As it came free in his hand, air burst out at pressure, bubbles swirling past him on the way to the surface.

He could see the man’s face quite clearly now, eyes bulging, teeth clamped together in agony. Quite suddenly he went over backwards in a graceful curve, like a leaf spiralling earthwards in autumn, the weight of his aqualung taking him down.

Mallory struck up towards the surface, chasing his aqua-mobile, which was rising slowly. He grabbed the handles and switched on, already aware of further shock-waves rippling through the water, bouncing from his body.

The aquamobile surged forward, helped by the turning tide. The sea-bed started to shelve again, and, below, he was aware of the pale forest of seaweed, the ribbons of black rock that were the beginnings of the reef.

Once again he was aware of a shock-wave curling around his body and he glanced to his right. Perhaps fifty yards away and coming up fast through moon-drenched water was a large underwater scooter, at least twice as long as his aqua-mobile, a frogman trailing behind.

Mallory kicked desperately, urging the aquamobile forward. And then the rocks swarmed up out of the gloom on either side and he rushed into the darkness of the Middle Passage. He switched on his spot, planing down to avoid the over-hanging roof. He was aware of a muffled throbbing in his ear and glanced back. A wide band of diffused light spreading through the mist told him that his pursuer wasn’t far behind.

He passed through a section where moonlight streamed in through cracks and fissures in the roof and knew that he was about half-way along the passage, somewhere above the wreck of the freighter. As he came into the clear section he plunged down and at ten fathoms a tapering steel mast loomed out of the gloom. Mallory held on with one hand and waited.

The darkness moved in on him with a terrible, suffocating pressure and the mast seemed to move a little as if the old freighter had rolled. He remembered the dark companionway, dead men’s bones crushed under a steel girder, and shivered, suddenly aware of the cold.