‘The crystals, look.’ Like the walls of the sarcophagus chamber, those set into the altar’s face had started to glow. These, however, were in different colours, reds and greens and yellows as well as blue-white.

The sight instantly brought to mind something more modern: a control panel . . .

‘Keep moving,’ Harhund ordered. She gave the altar a last uncertain look, then continued upwards.

Vorster and the mercenaries detached the sarcophagus from the ropes and carefully carried it down to the passage below. Nina followed, looking back at Eddie, under guard on the mausoleum stairs. ‘See you soon, love,’ he called to her.

She wished she had his confidence. ‘I hope so! I’ll—’

She broke off and grabbed the wall for support as the fortress trembled. One of the men carrying the sarcophagus slipped and fell, the others barely catching it before it crushed his leg. ‘What was that?’ snapped Harhund.

‘The iceberg’s starting to melt!’ said Vorster, recovering. ‘It’s full of fault lines – and they’re breaking apart.’

‘I guess you’d better work fast,’ Nina told Harhund, but she wasn’t sure Vorster was right. The shudder hadn’t felt like a tremor shaking the fortress. It seemed more like a vibration, as if the structure itself were the cause.

The gold and crystal coffin was secured to the next set of ropes, then the ascender started to haul it up to the entrance. The crystal ribs were now aglow, giving the corridor the look of a neon-lit funhouse.

Harhund brought the prisoners past the sarcophagus and signalled for Vorster to join them. ‘Wintz, go back to the boat,’ he told one man. ‘Bring it to the bottom of the cliff under the entrance. We will lower the coffin down to it.’ Wintz scrambled ahead.

The fortress shuddered again, everyone stopping to brace themselves. Nina heard creaks and groans from all around, as if the structure itself was straining against its frozen prison. Were the glowing crystals melting the ice? She raised a hand to a rib. No heat, just light. That wasn’t the cause – so what was?

Harhund spoke to Roche and another man, then faced his team. ‘I will take Professor Wilde and the key straight to the ship,’ he announced. ‘Once the coffin is aboard the boat, bring these prisoners.’

‘What about Eddie?’ said Nina.

‘If it is safe to take another coffin, he will come with it.’ The blond man paused at another tremor. ‘If it is not, we will all get out.’

‘We’re supposed to bring at least three coffins,’ one of the mercenaries objected.

Harhund fixed him with a patronising stare. ‘Do you want one of them to be yours?’ He looked back at Nina. ‘Now move. Quickly.’

They soon reached the entrance tunnel, blue light seeping through the open hatch. Under Harhund’s watchful eye, Nina retrieved and donned her crampons before stepping onto the uppermost ice ledge.