At the first lot of caves, the occupants of the earlier boats were diving merrily in or climbing carefully down ladders, their snorkels on their heads. Dora wished she were braver, that she didn’t view snorkelling as a terrifying endurance test. And as for scuba diving! It was her idea of hell.

They passed the landmark rock that stood alone in the sea with a natural archway, too small even for a boat to pass through, nosed round the headland and motored towards a gaping hole in the cliff, where their skipper slowed and threw down an anchor. ‘Now you swim,’ he announced.

‘Swim?’ almost shrieked Dora.

‘Come, I will show you,’ Ariadne giggled, searching on the floor of the boat for a mask and snorkel tangled among the ropes. ‘I am a good swimmer.’ She leapt joyously over the side, leaving Dora, in her off-the-shoulder Ralph Lauren swimsuit, to climb gingerly down the ladder.

‘I’ll keep an eye on the boat,’ offered Karl. ‘Make sure it’s still here when you come out.’

‘Karl,’ Dora instructed sternly. ‘If I can do it, so can you. It’s important!’

Reluctantly, Karl stripped down to extremely baggy trunks and dived over the side.

‘Don’t go away!’ he called to their skipper, who laughed as though the idea were a huge joke.

Once in the cave, Dora tried not to panic. As long as she could see the sunlight at the cave mouth, she told herself, she would be all right. If she looked down she could see the pattern of the currents on the sand of the seabed, as clear as the sharp lines of a woodcut. Above her, the rock was striped in greys and yellows by thousands of years of buffeting by stormy seas. Dora could appreciate its beauty while at the same time battling with terror.

Was this the right cave, anyway? Her certainty began to ebb away like the receding tide. And then she saw the side passage leading off to a smaller cave, and knew it was the place after all. ‘There!’ she shouted. ‘I saw something in there!’

Ariadne, braver than the other two, instantly dived down and out of sight as Dora held her breath.

‘Are you all right?’ asked Karl, sensing her fear.

‘Fine, but I’ll be better when we’re back on the boat.’

Ariadne seemed to be out of sight for far too long, and Dora felt her chest began to tighten with panic when the girl’s face appeared above the water. Dora knew at once that the cave was empty. ‘Perhaps I missed it,’ Ariadne began. ‘I got a bit scared. I will go again.’

Dora pulled herself together. She couldn’t let Ariadne take the risk. ‘Let’s go back to the boat. I probably imagined it.’

They swam gratefully towards the light. When they came out of the cave mouth, they looked round for the boat.

It was nowhere to be seen.