Garcia stared at Alex, surprised. Then he slowly nodded.

Five minutes later, Alex stood on the deck dressed only in shorts and a T-shirt, with an oxygen cylinder strapped to his back and two respirators-one to breathe through, the other spare-dangling at his side. He would have liked to put on a wetsuit, but he hadn’t been able to find one his size. He would just have to hope that the water wasn’t too cold. The BCD he was wearing was old and it was too big for him, but he had quickly tested it and at least it worked. He looked at his instrument console; pressure gauge, depth gauge and compass. He had 3000psi in his air tank. More than he would need. Finally, he had a knife strapped to his leg. He probably wouldn’t use it and would never normally have worn it. But he needed the reassurance. He went over to the side of the boat and sat down.

Garcia shook his head disapprovingly. Alex knew he was right. He was breaking the single most critical rule in the world of scuba-diving. Nobody ever dives alone. He had been taught scuba by his uncle when he was eleven years old and if Ian Rider had been here now he would have been speechless with anger and disbelief. If you get into trouble-a snagged air hose or a valve failure-and you don’t have a buddy, you’re dead. It’s as simple as that. But this was an emergency. Turner and Troy had been gone for forty-five minutes. Alex had to help.

“You take this,” Garcia said suddenly. He was holding an out of date dive computer. It would show Alex how deep he was and how long he had been down.

“Thanks,” Alex said. He took it.

Alex pulled his mask down, pushed the mouthpiece between his lips and breathed in. He could feel the oxygen and nitrogen mix rushing into the back of his throat. It had a slightly stale taste but he could tell it wasn’t contaminated. He crossed his hands, holding his mask and respirator in place, then rolled over backwards. He felt his arm knock against something on the side as the world spun upside down. The water rushed up to greet him and then his vision was pulled apart like a curtain opening as he found himself plunging into the water.

He had left enough air in the BCD to keep him afloat and he made one last check, getting his bearings on the coastline so that he would know where to swim to and, more importantly, how to get back. At least the sea was still warm, although Alex knew that, with the sun rapidly setting, it wouldn’t be for long. Cold is a dangerous enemy for the scuba-diver, sapping the strength and concentration. The deeper he went, the colder it would get. He couldn’t afford to hang around. He released the air from the BCD. At once the weights began to drag him down. The sea rose up and devoured him.

He swam down, squeezing his nose and blowing hard-equalizing-to stop the pain in his ears. For the first time he was able to look around him. There was still enough sunlight to illuminate the sea and Alex caught his breath, marvelling at the astonishing beauty of the underwater world. The water was dark blue and perfectly clear. There were a few coral heads dotted around him, the shapes and colours as alien as anything it’s possible to find on the earth. He felt completely at peace, the sound of his own breathing echoing in his ears and each breath releasing a cascade of silver bubbles. With his arms loosely folded across his chest, Alex let his fins propel him towards the shore. He was fifteen metres down, about five metres above the sea bed. A family of brightly coloured groupers swam past him; fat lips, bulging eyes and strange, misshapen bodies. Hideous and beautiful at the same time. It had been a year since Alex had last gone diving and he wished he had time to enjoy this. He kicked forward. The groupers darted away, alarmed.

It didn’t take him long to reach the edge of the cliff. The sea wall was of course much more than a wall; a seething mass of rock, coral, vegetation and fish life. A living thing. Huge gorgonian fans-leaves made of a thousand tiny bones-waved slowly from side to side. Clumps of coral exploded brilliantly all around him. A school of about a thousand tiny silver fish flickered past. There was a slither of movement as a moray eel disappeared behind a rock. He glanced at the dive computer. At least it seemed to be working. It told him he had been down for seven minutes.

He had to find the entrance to the cave. That was why he was here. He forced himself to ignore the colours and sights of the underwater kingdom and concentrate on the rock face. The time he had spent taking his bearings before the dive paid off now. He knew more or less where the tower at the Casa de Oro stood in relation to the boat and swam in that direction, keeping the rock wall on his left. Something long and dark flashed past high above him. Alex saw it out of the comer of his eye but by the time he had turned his head it was gone. Was there a boat on the surface? Alex went down another couple of metres, searching for the cave.

In the end, it wasn’t hard to find. The entrance was circular, like a gaping mouth. This impression was heightened when Alex swam closer and looked inside. The cave hadn’t always been underwater and over a period of time-millions of years-stalactites and stalagmites had grown, needle-sharp spears that hung down from the ceiling and protruded up from the floor. As always, Alex was unable to remember which was which. But even from a distance there was something menacing about the place. It was like looking into the open mouth of some giant, undersea monster. He could almost imagine the stalactites and stalagmites biting down, the whole thing swallowing him up.

But he had to go in. The cave wasn’t very deep and apart from the rock formations it was empty, with a wide, sandy floor. He was thankful for that. Swimming too far into an underwater cave, at sunset, on his own, really would have been madness. He could see the back wall from the entrance-and there were the first of the metal rungs! They were dark red now and covered in green slime and coral, but they were clearly man-made, disappearing up the far wall and presumably continuing all the way to the top of the Devil’s Chimney. There was no sign of Turner or Troy. Had the two agents decided to climb up after all? Should Alex try to climb after them?

Alex was about to swim forward when there was another movement just outside his field of vision. Whatever he had seen before had come back, swimming the other way. Puzzled, he looked up. And froze. He actually felt the air stop somewhere at the back of his throat. The last of the bubbles chased each other up to the surface. Alex just hung there, fighting for control. He wanted to scream. But underwater, it isn’t possible to scream.

He was looking at a great white shark, at least three metres long, circling slowly above him. The sight was so unreal, so utterly shocking, that at first Alex quite literally didn’t believe his eyes.

It had to be an illusion, some sort of trick. The very fact that it was so close to him seemed impossible. He stared at the white underbelly, the two sets of fins, the down-turned crescent mouth with its jagged, razor-sharp teeth. And there were the deadly, round eyes, as black and as evil as anything on the planet. Had they seen him yet?

Alex forced himself to breathe. His heart was pounding. Not just his heart-his whole body. He could hear his breath, as if amplified, in his head. His legs hung limp beneath him, refusing to move. He was terrified. That was the simple truth. He had never been so scared in his life.

What did he know about sharks? Was the great white going to attack him? What could he do? Desperately, Alex tried to draw on what little knowledge he had.

There were three hundred and fifty known species of shark but only very few of them were known to have attacked people. The great white-carcharodon carcharias-was definitely one of them. Not so good. But shark attacks were rare. Only about a hundred people were killed every year. More people died in car accidents. On the other hand, the waters around Cuba were notoriously dangerous. This was a single shark…