‘The lake is around two hectares in area. In water this cold, we won’t stand a long decompression, so let’s avoid it by staying shallow and surveying what we can within the no-decompression limit.’

‘But we need to get to the bottom,’ complained Boerman.

Nash brushed the dirt aside and placed his stick on the rock.

‘Limestone is white. It should easily contrast with something as large as a submarine.’ He looked up at Sura. ‘What’s its length?’

‘It’s 120 metres,’ replied Sura promptly.

Nash frowned. ‘That’s more than twice as long as a U-boat.’

‘Stick with the task at hand,’ she snapped irritably. ‘What visibility will you have?’

‘More than enough.’

That was a lie, because with these inadequate lights, the best they could hope for was 80 to 100 metres. Nash hesitated as something else occurred to him. Somewhere below them, the Hoosenbeck was finding its way out of this cave. It would be dangerously powerful, easily enough to suck in an unwary diver.

‘You were about to say something, Mr Nash?’ Her almond eyes were full of suspicion, and when he did not reply, she added balefully, ‘If anything happens to Jaap, you have my word that I will fly straight back to Hufi and deal with Frank Douglas and Mia Carter. Do you understand?’

On a ledge below the cave mouth, Nash strapped on the rebreather and turned on his dive lights.

‘Stay close, dickhe—’

Stepping into the water first, Boerman’s voice died as cold shock sucked the air out of his lungs. Nash followed, and the nine-degree water was like a cleaver splitting his skull. For several seconds, his ear canals felt full of boiling water.

Descending to thirty metres, the only visual reference in the void was the blank wall of rock behind them, and the refracted glow of the cave mouth above. Their first pass was along the left-hand side. The water was crystal-clear, and the rock wall beside them plunged well over eighty metres until it vanished. There was no sign of the bottom, just a blue-green haze as the limits of visibility were reached.

Most so-called ‘bottomless’ lakes were rarely more than 100 metres deep, but there were exceptions. Boesmansgat, a bottle-shaped cavern in South Africa, was a shade under 300 metres. Zacatón, where Exley had died, was around 320 metres. The recently discovered Hranická Propast, located in the eastern Czech Republic, was reputed to be an awe-inspiring 400 metres deep. At such depths, even a minute of bottom time would require something like fifteen hours of decompression, not to mention managing HPNS – high-pressure neurological syndrome – with its associated tremors, dizziness and reduced acuity.