‘The key!’ she suddenly cried, darting to the ship’s side and looking down. The artefact was the cause of all this – and it was bobbing somewhere below. The surface was awash with floating ice. Where was the key? If there was any chance that the murderous Harhund might reclaim his prize, she had to stop it—

Yellow amongst the white and blue. The pouch was drifting towards the cavern’s mouth, pushed by the rebounding waves.

Cheng still wore his life vest. ‘Give me that!’ she said, hurriedly pulling it from the surprised student. She put her arms through the straps – and leapt over the side.

She was already cold, but the freezing water was an even greater shock than before. The vest butting under her chin, she shook stinging brine from her eyes.

The pouch was nowhere in sight. She started to swim, hoping the exertion would counter the draining chill gnawing at her limbs. ‘Cheng! Where’s the key?’

Cheng gawped at her before looking hurriedly about, then pointing past the ship’s stern. ‘There, over there!’

Nina still couldn’t see the pouch, but she struck out regardless.

An almost impossibly deep rumble reached her, the vibration coming as much through the water as the air. The whole iceberg was straining as its weight began to roll it back over. A wave-smoothed outcrop of cyan ice near the entrance had been raised higher above the surface; now it sank again.

And the cave’s mouth was shrinking too, slowly retreating into the water.

The realisation galvanised her to swim harder, determined to find the key – while she could still escape with it.

Imka took the lead from Eddie as they descended into the Dionysius’s dark bowels; the South African knew the ship’s layout, and she still had a flashlight clipped to her harness. ‘In here.’

She pushed open a hatch and stepped into the engine room. Eddie recoiled at the stench of oil, seeing the floor slick with dark, glutinous ooze. Even at freezing temperatures, some of the engine’s lubricants had remained semi-liquid, leaking from the machinery while the ship hung inverted. ‘Will it still run without all this?’ he asked.

‘I don’t know if it will run at all!’ Imka replied. The main diesel engine, a hulking dull-green block the size of a car, filled the room’s centre. ‘I’ll use the compressed air system to move the pistons, but if there isn’t enough air in the tanks, there’s nothing I can do. There won’t be any power to start the compressors.’

‘You sure?’

‘I smelled acid when we passed the battery room. It must have drained out when the ship turned upside down.’

‘Glad we weren’t barefoot, then. Where’s this air system?’