Suddenly, a series of concussions thumped against the bottom of the Jet Ranger, as loud as a foot pedal on a bass drum. The crowns of several trees swayed and crashed down in a cloud of leaves. Boerman was using plastic explosives. Hartono cleared the leaves and dust away as he descended. Most of the trees had fallen down the slope. He settled the chopper on a ledge close to the entrance.

‘Kill it, Ricki,’ instructed Sura. ‘And stay here with the chopper.’

The mouth of the Hoosenbeck Cavern was a semicircular arch about the size of a three-storey building. That a mighty river had once flowed through here was beyond doubt. The rock had been carved by chemical processes and boulders ejected from inside the mountain. Enough light penetrated the interior for them to see that a couple of metres below the lip was an expanse of emerald-green water.

A vibration passed through Nash’s body. He’d been dreaming of this for half a lifetime – ever since first reading about the Hoosenbeck while sitting on his parents’ couch. But the moment was tinged with loss. It wasn’t just the context of a gun pointed at his back. It was because he felt unfit to enter the element to which he had dedicated his life.

‘Come on,’ said Sura. ‘We don’t have time to waste.’

The vastness of the space was intimidating. The lake could not be encompassed by the light of their powerful torches, but what they could see was as long as a football field. According to the 1960s explorers, it reached twice as far again into the bowels of the mountain. Nash visualised its creation: the millennia of acidic snowmelt finding its way down some fault in the Miocene limestone and steadily eroding this vast dome. He realised the Hoosenbeck both entered and exited the lake below the surface. At some point, around two hectares of unsupported roof would collapse, and the gorge would lengthen again.

Nash allowed himself to simply absorb the magnitude of the place. Analogies and allusions swam into his mind. Surely it was the hall of the mountain king which Tolkien had visualised in Middle-Earth? Or was it Dante’s Ninth Circle of Hell?

Then the lines came to him unbidden:

In Xanadu did Kubla Khan

A stately pleasure-dome decree:

Where Alph, the sacred river, ran

Through caverns measureless to man

Down to a sunless sea.

The poetry conjured up his last journey with Natalie. En route to the Octopus, Nash had been musing on the final sentence in the foreword of his book: For what we discover in caverns unknown to man is nothing less than the potential of human imagination itself.

‘It’s almost there –’ Natalie had her eyes on the undulating dirt road – ‘but you need to drop the bastardised allusion.’

‘You’ve got a problem with Samuel Taylor Coleridge?’

She punched him lightly on the arm and he grinned.