Mackenzie’s chest felt like it was going to burst, and his legs were leaden. He wasn’t sure how he was going to make it through this tunnel if this went on much longer . . . And if the current was this fierce going into the tunnel, how could they possibly push against it on the way back out?

Finally he shot out into a peaceful bubble of water. His fins met rocks below him and he was able to stand up, chest-deep in water, his headtorch lighting the way.

The cave was cramped and arched, carved by centuries of high tides. Geologically, this was the beginning of what would one day become another Devils Kitchen. But for now it was a round cavern with a swirl of gravel rising in the middle of it, volcano-like. On that small rise sat a thin, shivering boy, arm raised against the blinding torchlight.

All through the space, the drone echoed and overlapped, sounding eerily like singing.

Mackenzie spat out his regulator. ‘It’s okay, Forest! We’re here!’

‘M-Mack?’ Forest pulled himself to his feet. ‘Is it . . . Are you . . . really here?’

‘Mack, look at the water,’ came Ahab’s harsh croak. His teeth were chattering from the cold, but there was no doubting the urgency in his voice.

Mackenzie saw what had distressed him. The water around them was beginning to swirl. It was still slow, but it wouldn’t take long before the filling cavern became a whirlpool.

Mackenzie, desperate to get the scuba gear on Forest, stumbled up the hill towards him. The boy threw himself into Mackenzie’s arms.

Ahab staggered up beside them, taking the spare scuba gear off Mack. ‘There’s a good lad. You’re doing great,’ he said to Forest. ‘We’ve got to get this gear on you, but you’re gonna be fine.’

The singing-drone came to a crescendo, and a giant wave exploded out of the tunnel, completely submerging all three of them. Mackenzie clung to the others, rooting himself in the gravel with his fins.

It seemed like hours but could only have been seconds – finally, the water receded. All three of them gasped for breath.

‘Mack! The gear!’ coughed Ahab.

Mackenzie saw that his hands were empty.

All the spare scuba equipment was gone. Washed into the dark water of the cavern.

‘We have to find it!’ said Mackenzie.

Ahab turned once on the spot, then looked Mackenzie in the eyes. ‘We don’t have time.’

‘He’s not getting out without it!’

The singing-drone grew louder again, but this time they were ready for the surging water, bracing against each other until it passed.

‘We’re going to die here,’ spluttered Forest.

‘Not all of us,’ said Ahab. He began unstrapping his BC. ‘You and Mackenzie are the ones with the future.’

‘No,’ said Mackenzie.

‘Yes, son!’ said Ahab. ‘Get yourself ready, before another wave comes.’

‘No!’ shouted Mackenzie. ‘We’re not leaving you!’

‘Yes,’ said Ahab. ‘You are.’