‘No, Rob, I’ve got a problem with the fact your hero already used that line in the title of his book.’

‘Caverns Measureless to Man,’ Nash recited sonorously, ‘by the late Sheck Exley, America’s greatest cave diver. Tragically perished below 300 metres in a sinkhole at Zacatón, Mexico, 1991.’

Natalie frowned. ‘I wish you wouldn’t do that. Especially before a dive in the Octopus.’

Citing fatalities in the vocation he lived and breathed was Nash’s way of acknowledging those who’d gone before and learning from their mistakes, but it was poor timing. He put his hand on Natalie’s smooth brown thigh.

‘You’re right. I’m sorry.’

She blew him a kiss. ‘I agree it is a perfect description for the lure of cave diving – especially if you happen to be a man! But I’m sure you’re smart enough to come up with another.’

‘OK.’ He smiled at her. ‘How about, “Because it’s there?”’

‘Idiot!’

Those seductive creases at the corners of Natalie’s mouth, the delicious peal of her laughter . . . she was still there inside his head! For a moment Nash was overwhelmed. Then a prod from Boerman’s pistol expunged the vision, and he was grateful for the darkness to hide his pain.

Their footsteps echoed mournfully, as Sura led them on a circumnavigation of the still lake. Mineral deposits on the walls indicated the ledge upon which they were walking would be a couple of metres underwater in the wet season. But it was another faint ring, a good five metres higher, which caught Nash’s eye. Once the lake had been far deeper, but only for a short span of years, just long enough to leave the ghost of its presence.

Towards the back of the cave, the ledge sloped down into crystal-clear water. Sura gasped as she stepped into it. The shock of the icy snowmelt, not much more than nine degrees Celsius, was like wading through fire. For several minutes they proceeded at thigh-numbing height, until Boerman cried, ‘Look up there!’

In the beams of their flashlights, a strange brown object drooped from the rock above. Like an oversized jungle vine or root, it was several metres long and as thick as a man’s arm – a rusted steel hawser, grotesquely swollen as oxides cannibalised the integrity of the metal. The shackle was held in place by four heavily corroded bolt heads protruding from the limestone.

Boerman gave a triumphant whoop and the sound echoed brutally around the cavern. For Nash, it was heresy. Aside from the slow plink of dripping stalactites, and occasional shrieks of bats, these lost worlds were worlds of silence, demanding veneration.

‘Look where the water level was when they drilled those bolts.’ Boerman turned to Sura excitedly. ‘It must be right here below us!’

Together they directed their flashlights at the water. It was gin-clear, the rocky sides of the cave clearly visible, tumbling down into unseen depths.