Somehow Sura had swallowed a cock-and-bull story about sunken treasure. Nash exhaled softly. It was a common aberration. Because for an inert metal, gold had a corrosive effect on common sense. Nash recalled his second expedition to Mexico’s Timoche Cenote – a giant freshwater sinkhole – while trying to complete a fifteen-kilometre connection to the Caribbean Sea. Navigating a maze of interconnecting tunnels, his team had come across a deposit of gold Mayan artefacts in a minor cenote. Nash had surrendered these to the Mexican authorities, but two American divers went to buy metal detectors. Nash had left them to it and made the breakthrough to the coast himself. Two weeks later, the Americans’ bodies were found in a burned-out van by the side of the road. A local gang had tortured them. Later, someone had blown up the cenote while trying to remove a plug of ancient trees and debris, closing down that part of the system for good.

In the darkness, he could feel Sura staring at him.

‘You said this lake is one to two hundred metres deep.’

‘I’m a prisoner, not a consultant.’

Nash was risking another fist, but pushing back was the only way to get information which might lead to a way out. Sura shone her flashlight directly into his eyes, making him squint.

‘Must I again remind you that your life, and those of your friends, depends solely on your value to me?’

‘How do I know they’re alive? I need proof.’

‘You’ll get it soon enough. Now, the depth, Mr Nash?’

‘Put a line down and find out.’

‘Did you bring enough rope?’ she asked Boerman.

‘We don’t need it . . .’

‘Very well,’ she sighed. ‘Let’s get the gear.’

When Boerman dragged two rebreather units from the chopper’s cargo cradle, Nash was incredulous. He’d assumed they were flying back to the Albany to equip themselves.

‘You can’t be serious. Without drysuits we’ll freeze down there.’

Boerman glowered. ‘It’s a preliminary reconnoitre. Grow some balls, man.’

Sura’s eyes narrowed. ‘You have a better idea?’

‘Yes. Properly equipped, we can survey safely and efficiently.’ Nash explained that hypothermia was an insidious killer that crept up on you. ‘In water this cold, our dexterity will be compromised in under five minutes. Within thirty to sixty minutes, we’ll lose consciousness and drown.’

‘It doesn’t sound like an exact science to me,’ Sura said doubtfully.

He explained the wide variable was reflective of individual physiology and size. Boerman would last longer because his huge bulk shed heat more slowly.

‘Sura, I’m telling you this is a dangerous waste of time. You can see for yourself the size of the lake –’

‘Says the master of wasting time,’ Boerman scoffed. ‘Let’s do this.’

Sura nodded. ‘Jaap, cut him free.’

In a last-ditch attempt to prevent Boerman killing them both, Nash hastily proposed a plan scratched out in the dirt of the cave mouth.