The light in the tunnel took on a sapphire hue as it cut through layers of dense ice. Ahead, the cave began to widen out. ‘We’re almost inside,’ said Imka, turning her head to search for the Dionysius. ‘Where is it? It was on the radar, we should be able to see it!’

Vorster used a walkie-talkie to call Tate aboard the Torrox. ‘Al, how deep in were those radar signals?’

‘Hard to say exactly,’ came the crackling reply, ‘but less than two hundred metres. The radar can’t penetrate ice any deeper than that.’

‘Then where is it?’ Imka demanded in frustration.

Nina looked past her. The cavern became discernible as her eyes adjusted to the low light. The rear wall was well over a hundred metres away, but there was nothing there except ice. The wall to her left curved upwards out of the water until it became practically vertical, a sheer swathe riven by cracks and protrusions; at its far end was an ominous field of spikes. Icicles once hanging from the ceiling now stabbed upwards after the iceberg’s roll. The right side of the cave narrowed to nothingness.

The water inside was calm – and empty. So where were the objects on the radar?

The answer came as they cleared the tunnel and entered the ice cavern proper.

The first thing Nina spotted made her blink in surprise at its incongruity: a ship’s anchor, dangling from its chain about forty feet above. But surprise turned to shock as her gaze followed the chain upwards – and saw it was still attached to its vessel.

The Dionysius was embedded in the cave’s ceiling almost a hundred feet over their heads.

The survey ship hung inverted, loose lines and chains hanging from it like vines. Metal and wood alike were coated in frost and ice. A ghost ship, suspended precariously from the roof of its frozen tomb.

Everyone stared in amazement at the gravity-defying sight.

‘Oh my God,’ Imka gasped. ‘Look at that!’

‘Definitely not something you see every day,’ said Eddie. ‘Even for us!’

Naider stopped the engine, the RIB coasting slowly into the cave. ‘How did it get up there?’

‘It must have gotten wedged in the ice,’ Nina said. ‘When the iceberg rolled over, it went with it . . . and then froze in place.’

‘Not frozen enough,’ Eddie observed. The glassy blue ceiling was split by numerous cracks around the Dionysius, icy fragments dropping in a faint but continual rain as the ship’s weight pulled ceaselessly at its prison. With the outboard shut down, they could also hear noises from above. A low, creaking rumble formed a rolling bassline to intermittent cracks and bangs. ‘It’s going to break loose, and probably sooner rather than later. I don’t want to be under it when it falls!’

Naider made a hurried course change to angle the boat out from beneath the suspended ship.