They sat on a narrow outcrop above the gravel beach and got ready. Mackenzie pulled on his BC, mask, headtorch. They couldn’t have brought wetsuits too, it would’ve been far too heavy – they’d just have to make it work without them. They’d brought a length of diving rope and he used that to tie Forest’s gear to his front, trying to balance out the weight of his own gear on his back.

Mackenzie eyed the ocean, whipped into crosshatch waves by the Black Wind. It was worse than it had been that day in Devils Kitchen, filling him with dread. The water rolled up over the beach. It would suck the feet from under them if they weren’t careful.

Ahab gripped Mackenzie to help him balance while he pulled on his fins. Then Mackenzie returned the favour.

When they were ready, the two men grasped hands.

‘Black Wind at morning, sailors take warning,’ said Ahab.

They jumped into the hungry ocean.

Instant chaos. The pull of the water, the agony of crushing pressure and howling waves. Freezing cold, salty, loud. Mackenzie was flipped upside down, his side bashing into rocks, but he kicked and kicked and kicked, deeper and deeper, releasing buoyancy . . .

Breathe in, bubbles out.

Finally he reached a depth where the current wasn’t pulling him in the wrong direction, but the water was darkly twilit. Mackenzie switched his headtorch on and shone it around. Kelp and sand and silt swirled around him.

Close by, an answering beam. Ahab’s figure loomed out of the muck.

Mackenzie felt a surge of unease – how could the current be so strong? What hope did they have?

Ahab swam closer, making the okay? gesture.

Mackenzie responded in kind.

He turned and kicked, sweeping the base of the cliff with his torch. He looked for places where the patterns in the silt suggested calmer water, at the same time scanning for the entrance to Siren’s Cave.

He found it before long. A maw of darkness in the wall of the cliff: the crevasse that led to the cave. He pointed it out to Ahab, and the two of them swam in, staying low, using the kelp to pull themselves forward.

The torchlight leading the way, they kicked and steered their way into the tunnel, fighting the surges that tried to flush them back out into open water. It was like being in a stone oesophagus – the current was breathing, and the kelp moved with each inhale and exhale.

When the water was being sucked out, Mackenzie had to kick like mad just to stay in place, and then when it rushed back in he was thrown forward dangerously fast. Sometimes Ahab slammed into him, caught up in the current and out of control.

The tunnel was smaller than he remembered, but luckily the kelp made it slippery and soft. Otherwise they’d have been torn to shreds by the rocks.