Weighed down with gear, they waddled into the cave and clambered down to the entry ledge, where Hartono had positioned the heavy-duty scooters.

The lithium-battery-powered tubes looked something like a stubby torpedo with a covered propeller. Capable of propelling a diver at speeds of 100 metres a minute for up to eight hours, they extended range and offset fatigue by a huge margin.

‘Take it slow at first.’ Nash made a point of warning his rookie offsider. ‘These things will give you an embolism before you can blink.’

The Afrikaner waved him away. ‘Stop trying to run the show. I know what I am doing.’

Nash glanced pointedly at Sura. If her bad boy screwed up, it would not be his fault.

They waited while Hartono jumped into a small grey inflatable and motored out to the centre of the lake. He threw in a coil of weighted line, which they would use as a guide and for decompression stops. Nash silently counted the seconds as the loops slipped into the water, and his bowels began to clench.

Christ, this thing is deep.

Eventually, Hartono yelled excitedly, ‘It’s down!’

‘Shit, man, what is the fokking reading?’ bellowed Boerman.

‘It’s at 203 metres,’ echoed the reply.

Boerman grinned. ‘We’ll spot it easily.’

Sura looked less confident on her rocky perch.

‘Mr Nash, repeat the plan.’

‘We’ll descend until we see the bottom and no further. That will be around 135 metres or so. We’ll sweep using the same pattern as last time.’ He looked meaningfully at Boerman. ‘No spontaneous descents. Got it?’

‘Ja, I heard you.’

Nash slid his legs into the loops of the scooter harness, pulled on his fins, and clipped himself to the machine. Easing off the ledge, they submerged slowly.

In contrast to the day before, Nash was warm and dry. Giving Boerman a thumbs up, he gently depressed the thumb speed control and cruised forward. Moments later, Boerman shot past like Wile E. Coyote on an Acme rocket, veering dramatically up and down, until he got it under control.

They steadily descended, switching from air to trimix, reducing the oxygen percentage to suit increasing partial pressure. At 100 metres they reached Hartono’s line and hovered in the green void. No sign of the bottom, nor the dinghy at the surface. It felt like outer space – the sensation was enhanced by the fact that rebreathers shed no bubbles.

They’d reached 140 metres when slabs of calved-off limestone appeared out of the gloom like giant footsteps stacked haphazardly. Nash arrested their descent with the pale limestone floor visible fifty metres below. There was no time to waste. Every minute spent at this depth would cost them twenty-three minutes of decompression.

At maximum speed, they set off. With the floor of the cave unfolding in the glare of their brilliant lights, it was like flying through the world’s biggest swimming pool, and Nash almost forgot the weight of water above him.