Stefano must have blacked out. When he came round, he was looking down on an agitated crowd. They were clustered around something lying on the dark, wet sand, just clear of the waves. At the centre of the gathering crouched a squat, middle-aged man in improbably tiny Speedos, buttocks in the air. Bent over the body of a pale young man, he was trying to give him the kiss of life. The poor bastard, thought Stefano uncharitably. He would get a shock if he came round to find that guy sucking his face.

Craning to get a better look, it occurred to him that the stricken swimmer's candy-striped trunks looked familiar. Now he realised that so was the body, which really was shamefully pallid compared to all the bronzed onlookers. It was superficial to be so critical, but Stefano had every right, because that was his own body he was looking down at. That was him, lying there on the beach.

This was like the weirdest dream he had ever had, but he somehow knew, as surely as he had ever known anything, that this was not a dream. This was happening.

He watched the kiss-of-life guy come up for air, puffing it in with a great gulp, and then hunch over for another attempt. The scene was silent, and it took Stefano a moment to grasp that this was not because nobody was talking, but because he could not hear. There must still be water in his ears after being pulled under like that. If only his rescuer would stop the kissing thing for a moment and tilt his head from side to side, it might drain out.

He could see the onlookers' mouths moving as they jostled inwards. Now someone – Stefano recognised the soft-drinks seller who hawked Coca Cola from a cart in the middle of the beach – pushed them back. Give him some air, he would be saying, because that was what people always said in situations like this, only he would be saying it in Spanish, and Stefano had no idea what that was. And there, finally, was Adam, with only a hint of anxiety at first, turning abruptly to panic. Stefano did not need to lip-read to know that his own name was being shouted.

Then, abruptly, the scene was gone. He could no longer see himself, nor any of the people around him, and he was surrounded by specks of white light. As his eyes adjusted to the brightness, he saw that he was in some kind of tube. Not that he could see the sides – the lightness was everywhere and nowhere – but it felt like some kind of extraordinary tunnel because it was brighter up ahead, and now he was conscious of rising towards the far end of it. He felt light-headed, euphoric, as if the whiteness were shining inside him. It reminded him of the first rush of a pill, in the days when they still worked and you knew why it was called ecstasy, only there was nothing illicit about this. What was happening to him? As he rose further and further up the long, white cylinder he was curious about what he would find at the end, but he felt no fear.

He blinked to focus better as the white walls rushed past him. There was definitely something up there now, different from the shining white, covering the entire end of the tunnel. His attention was drawn in particular to a pinpoint of pitch black at the centre, which gradually widened as he came closer. Now it became a deep, inky pit that he seemed to be moving steadily towards, a black hole at the end of the white corridor. All around the edges of this hole straggled translucent rivulets of white and grey, coursing over a glistening blue ground and pouring into the black abyss like water into a well. Beyond these shining trails of white, he saw, was something else: a border, like smooth rocks circling the pool of rivulets. Or maybe it was softer ... porous, organic, alive.

It looked like an eyelid, he suddenly realised: the black hole was a giant pupil, the straggling rivulets over a shining blue ground were the iris, and that giant lid must belong to ...

'He's breathing!' shouted someone, disturbing his concentration, and he heard a painful rasping sound accompanied by exclamations of relief. He blinked his eyes open, which was odd, because he could have sworn they were open already, and found he was looking at the kiss-of-life guy again, not from above this time, but from his own body, flat on the wet sand. He could hear again, that was for sure, and the rasping was coming from his own throat as he coughed up brackish sea water. The kiss-of-life guy smiled. With his wide, fat lips and gappy teeth, he reminded Stefano of a kindly toad. Then the face was replaced by an even more welcome one.

'I thought I'd lost you,' said Adam, and Stefano could see he was trying not to cry. 'How do you feel? Don't try to talk. We need to get you to a hospital. Can we get him to a hospital?'

Someone was shouting Spanish into a mobile phone. Adam kept turning away to talk to people, and it seemed an age before he was paying attention to Stefano again and there was a chance to tell him. To try to tell him, at any rate – it was hard to breathe, let alone talk, and his whole head stung with salt.

'Don't try to speak,' said Adam. 'Just stay there for the moment and then we'll get you to hospital. No, you mustn't try to move. Please, lie still.'

Stefano tried again to reach for Adam's hand, finding it this time and attempting to pull him closer. 'You don't understand,' he was saying in his head, only it would not come out like that. It would barely come out at all. But he kept trying, and Adam was eventually forced to stop shushing him and lean in close to make out what he wanted to say.

'I think ...' said Stefano at length, concentrating hard to make the words separate themselves from one another.

'Yes? What?'

'I think I just saw the eye of God.'

The holiday on this strange little volcanic island had been Adam's idea. They had started on the opposite coast, under a blanket of cloud. There were no beaches on that side, and the bad weather seemed so permanent that it was even cloudy in the postcards.

As the outermost part of the archipelago, the island had been celebrated in ancient times as the edge of the known world. This excited Adam. For someone who had always been interested in astronomy, the further discovery that this had once been the zero meridian, where the western and eastern hemispheres officially met, made the place all the more evocative.

Unfortunately, soaking up this conceptual geographical history was not Stefano's idea of a good holiday, especially when it rained ceaselessly on the day they trekked to the lighthouse that marked the western tip of the rock. There was not much to see, and Stefano was conspicuously unimpressed.

'There's not even a plaque,' he observed, after circling the lighthouse twice.

'What do you want it to say? This used to be the edge of the world and now it's not?'

'It feels like it still is.'

They heard the climate was better on the opposite coast, so the following day they were back in their hire car, threading across the mountainous interior. It took half the morning to wind around the ravines, up into the cooling mist of the ancient forest, and then to hairpin down into a terraced gorge, lush with date palms and banana trees, where it was a blazing eighty-eight degrees.

They found an apartment in the hippie-run resort at the bottom of the valley. It had a cushion-thin mattress on the bed and a door that took five minutes to unlock, but you could...