Instead, Boerman bent down and picked up a fist-sized chunk of limestone. He lobbed it into the jumble of logs and vegetation and they listened to it rebounding off timber and steel, until the sound of it shattering on the rocky floor echoed up. The Afrikaner whistled appreciatively.

‘That’s one deep hole.’

Sura looked quizzically at Nash. ‘I don’t understand. This sinkhole is not only deep, it is also dry. How can the Hoosenbeck possibly resurface just a few hundred metres downstream of this location if the two are connected?’

Nash gratefully seized the lifeline. ‘You can’t think in terms of straight lines of fall. Limestone gives rise to complex hydrological systems – multiple passages funnelling underground rivers which can flow intermittently, depending on rainfall. The discharge from the Hoosenbeck Cavern is currently bypassing this sinkhole.’

Sura’s eyes narrowed. ‘What do you mean, currently? Are you saying this sinkhole floods when it rains? That it is connected to the Hoosenbeck?’

He nodded quickly. ‘The Dutch speleologists would have encountered this sinkhole when they explored the valley. It must have been full at the time, or they would have discovered this submarine.’

‘They came in March, the end of the wet season,’ Sura murmured.

Boerman glanced up at the sky, now studded with plump white clouds.

‘And it’s due any time.’

‘Then we need to move quickly.’ Sura put her hand on the Afrikaner’s broad shoulder. ‘Jaap, run back up, fetch rope and two harnesses.’

‘Two?’ The big man paused.

‘Mr Nash is going to assess the situation with you.’ Before Boerman could argue, she snapped her fingers. ‘Hurry!’

The Afrikaner took off at a run and Nash breathed again. As long as the threat of inundation remained, his dive skills had currency.

Sura took out her pistol.

‘Sit down there, Mr Nash – yes, right by the edge – and please, don’t try anything stupid.’

She made herself comfortable on a fallen log and checked her phone for a signal. In the silence, he became aware of the faint sound of running water in the sinkhole far below. Yes, somewhere down there was his escape route.

Sura coughed and bared her little white teeth in a smile.

‘You’re an interesting man, Mr Nash. In many ways you remind me of myself.’

Nash raised his eyebrows.

This is going to be interesting.

‘We’re both dreamers who think big, and are prepared to take risks to achieve our goals.’

‘We are nothing alike,’ he promptly replied. ‘I don’t kill people for money.’

‘Oh, really? How do you explain what happened to your wife? Wasn’t her death a direct result of your ambition?’ Sura looked like a cat focused on its next meal. ‘It’s what drives you, isn’t it, Mr Nash? You deconstruct the mysteries of the earth to be the first.’