I tried to swim along the most likely route Christo would have taken between the cave and the beach, pausing every so often to climb a boulder and scan around me. By the time I'd crossed three-quarters of the lagoon I could see flashlights on the beach. The others had arrived, but I didn't call to them. I wasn't decided whether their distant presence was a reassurance or a drag.

Shadowed

Christo's name was being called. Low-pitched and high-pitched, boys' voices and girls' voices, floating across the lagoon. I didn't like the sound. From my position, resting on a boulder by the entrance to the cave, the call was always answered by an echo. It gave me the creeps, so I swam into the cave to cut the sound out. Then, once started, I didn't stop. I swam straight ahead until I bumped blindly against the rock-face where the passage ducked below the water-level, took a lungful and dived.

It was very exciting underwater. The rock walls, never warmed by sunlight, cooled and deadened the water. I felt as if I'd dared to enter a forbidden area, the zone I'd shied away from with Etienne and Francoise, diving for sand on Ko Samui. 'Braver now,' I thought dreamily, relaxing my legs and slowing my arm strokes. I wasn't hurried; Christo and the shark seemed rather distant concerns. I was almost enjoying myself, and I knew my lungs were practised enough to keep me under for over a minute thirty without serious discomfort.

Every few feet I stopped and groped around to make sure I wasn't accidentally heading down the side passage to the air pocket. In the process, I discovered the central passage was far wider than I'd previously imagined. At full arm's length I couldn't touch either of its sides, only the barnacle-covered ceiling and floor. I realized, with a reproachful grimace, that to have ended up in the air pocket I must have strayed quite a way off course.

I grimaced harder when I came up on the seaward side of the cliffs. A strong night swell gave me a harsh reality check, pulling me out of my otherworldly stupor by knocking me against the rocks. I had to clamber awkwardly out of the water, slipping on algae and cutting my legs yet again. When I'd found my balance I looked around for Christo and yelled his name, without a lot of hope because the moonlight was bright enough for me to see he wasn't there. I could, however, see the boat. It was floating freely in the small cove that served as its port and hiding-place, untied. I made my way over and scooped the rope out of the sea, securing the boat with as many granny knots as the rope's length would allow – not very nautical but the best I knew how. Then I perched on a small rock-shelf and wondered what I should do next.

The problem was, I could easily have missed Christo on several stages of my search, the boulders particularly. It was possible he'd already been found and was back at camp. But I also had a powerful sense that I hadn't missed him. The untied boat told me that they'd got as far as the entrance to the cave. If Christo hadn't been injured, he'd have made the swim with Karl. If he had been injured however, Karl would have left him where I was sitting, intending to come back for him later.

'Unless…' I muttered, clicking my fingers and shivering in the sea breeze.

Unless he'd been killed outright at sea, in which case it was a safe bet he'd never be found.

'Or…'

Or he'd only been injured a little. He'd been fit enough to make the swim through the underwater passage. He'd swum under with Karl, helping him with Sten, but something had happened. Swimming three men wide. Slightly hurt. Had to be scared and confused.

'That's it,' I said firmly.

Karl wouldn't have realized Christo had gone until he came up in the lagoon. With Sten to deal with, maybe still alive, he couldn't go back. Maybe he waited for as long as a man could last without breathing. One or two desperate minutes extra to be sure. Maybe then he gave up.

'That's it. Christo's in the air pocket.'

I stood up, filled my lungs, and dived back into the water. I found the side passage to the air pocket on my third attempt.

I surfaced, incredibly, into stars. I wondered if I'd missed the turning a fourth time, got disorientated, come up in the open sea or the lagoon. But the stars were beside and ahead of me. The stars were everywhere, unnaturally dense, within reaching distance and a thousand miles away.

Lack of oxygen, I thought, and took a tentative breath. The air tasted better than the last time, maybe freshened by an extra-low tide, but the stars didn't go away. I took another breath, shut my eyes, waited, opened them again. The stars remained, twinkling away, even a little brighter. 'Impossible,' I whispered. 'This makes no…'

A murmur cut me off, coming from somewhere in the thick constellation. I paused, treading water slowly.

'Here…' said a quiet voice.

I pushed my hands out and felt a rock ledge, then I ran my hands along and felt skin.

'Christo! Thank God! I've been…'

'…Richard?'

'Yes.'

'Help me.'

'Yes. I'm here to help.'

I continued feeling along the skin, working out which part of the body I was touching. It was surprisingly difficult to tell. What I first took to be an arm turned out to be a leg, and what I took to be a mouth turned out to be a wound.

Christo groaned loudly.

I shook my head. 'I'm sorry… Are you badly hurt?'

'…I have… some injury…'

'OK. Do you think you're able to swim?'

'…I do not know…'

'Because you have to swim. We have to get out of here.'

'…Out?'

'We've got to get out of the air pocket.'

'Air…pocket…?' he repeated, forming the sounds uncertainly.

'Air pocket. Uh… this little cave. We need to get out of this cave.'

'But sky,' he muttered. 'Stars.'

I frowned, surprised that he could see the stars too. 'No. They aren't stars. They're…' I hesitated. Then I reached up and my hand sunk into cold strands of hanging seaweed. 'Not stars,' I finished, managing a short laugh, and pulled down a glittering strand.

'Not stars?' He sounded upset.

'Phosphorescence.'

There was a small space left on the ledge so I hauled myself out of the water and sat beside him. 'Listen, Christo, I'm afraid we're going to have to try for this swim. There's no choice.'

No reply.

'Hey, did you get that?'

'…Yes.'

'So what we'll do is I'll swim ahead using my arms, and you'll have to hold on to my legs and try to kick. Are your legs injured?'

'…Not legs. It is my… my…' He felt for my hand and put it some place on his torso.

'So you can kick. We'll be fine. No sweat.'

'…Yes.'

His voice sounded like it was getting fainter so I talked my plans out loud to keep him awake. 'Now our only problem is going to be finding the right passage out of here. If I remember right there are four passages to choose from, and we don't want to get the wrong one. You understand?'

'…I understand.'

'Good. Let's do it, then.' I leant forwards to drop back into the water, but stopped myself just as I was about to drop off the ledge.

'What?' asked Christo feebly, sensing that something had happened.

I didn't answer. I was transfixed by a chilling and beautiful sight – a slender comet cruising through the blackness beneath my feet.

'What happens, Richard?'

'Nothing… There's just… uh… something down there.'

'The shark?' Christo's voice instantly rose to a frightened sob. 'Is it this shark?'

'No, no. Definitely not. Don't worry.' I watched the comet carefully. Actually, when I'd first seen it I'd thought it was the shark, which is why I'd hesitated before answering Christo. But now I was sure it wasn't. Something about the way it moved wasn't right; it wasn't gliding and it was too jerky. It was more like a person.