After perhaps fifteen kilometres the gorge narrowed to no more than 200 metres, and Hartono gripped the cyclic more tightly. Bouncing off the rocky ramparts, the Jet Ranger’s roar was an eerie ghost echo, which seemed to presage no good. Here, nature was losing her fight with stone. The dwindling trees crammed into two narrow strips had etiolated, their bereft crowns meeting above the water like hands clasped in prayer. They flew across a whole forest of huge skeleton trees – dead white trunks devoid of branches, suggesting a catastrophic dieback in the distant past. For a good kilometre, this continued until, dead ahead, the monolith dwarfed everything. It took Nash a moment to realise that this was the shadowy end of the gorge itself – a sheer black wall rising a thousand metres.

Forgetting his predicament, he eagerly craned his head down. Somewhere at its base, shrouded by tenacious trees, lay the mouth of the Hoosenbeck Cavern.

‘There it is!’ Sura pointed to the top of a wide arch and crescent; a hint of darkness beckoned within.

Hartono brought the Jet Ranger as close to the bastion as he dared, the tips of his carbon-fibre blades just metres from the rock. Gradually, he eased the chopper down, a few cautious metres at a time, but the downdraughts were playing havoc with his controls.

‘I need more margin,’ he told Sura. ‘It’s too fucking close.’

She leaned around to confer with Boerman. He put on a rucksack and picked up the coil of climbing rope.

‘Give me ten minutes.’

‘Make it five. We must conserve fuel.’

Hand on the door release, Boerman stared at Nash.

‘I don’t trust this fok.’

Sura took out a small pistol and pointed it at Nash’s forehead.

‘This is a low-velocity cartridge. Rest assured, if you try anything, it will not damage the helicopter when I shoot you.’

When Boerman opened the rear door, the volume was deafening. Chilled air rushed in, bearing the smell of wet rock and decaying vegetation. With boots planted on the skid, Boerman swiftly tied off one end of the rope and flung the remaining slack towards the ground. Then, gathering a loop, he connected it to the karabiner on his belt and attached a cammed descender above it. Reversing position, he gave Nash a grin and fell back, dropping for a couple of seconds before engaging the descender. Hartono swore as the chopper bucked and swayed under the impact. Within seconds, Boerman touched down, detached himself from the rope and was running for the nearest tree.

It was then that Nash noticed something very strange. What had become of the mighty Hoosenbeck they had just flown up? All he could see was a modest stream bubbling out of the cave and trickling down the wide and dry river bed in a pale imitation of the force that had carved it.