He must be some sort of military experiment, she thought. Aimee drew in a cold draught of fresh air and smiled. As a spotlight of real sunlight hit the tunnel floor, she realised it didn’t matter. It was enough for her to know he was special. He had risked his life for all of them, dozens of times. He had saved them.

Lieutenant O’Riordan was the first to spot the dark debris appear out on the ice.

The three helicopters hovered over the almost circular bowl-shaped hole in the ice. Just under forty feet across at the top, it widened at the bottom to a belly of approximately fifty feet. This was due to the heat effect of the thermite being concentrated downwards and sideways the further it sank into the ice. The edges were glass smooth as if the hole had been polished by a thousand jewellers, and now that the surface snow was removed the outline of the city could be seen under the ice. The only blemish was a football-sized piece of dark rock and its exit crater towards one side of the ice hole.

The helicopters landed back from the edge and immediately disgorged men carrying an array of climbing equipment. Drop winches were pinned to the upper ice lip and the men quickly rappelled down into the crater.

Water was just starting to run down the sides of the large bowl and the air at the bottom was balmy and very humid. It wouldn’t be like that for long; the bottom of the pit would fast fill with warm water and once that cooled to slush it would block the hole. From then it would set like stone.

O’Riordan and his men charged towards the hole with ice axes and a single portable pneumatic jackhammer.

The initial flush of water never abated and quickly became a torrent as the melted ice poured through the hole Alex had opened. The good news was that the liquid was still warm and it was smoothing and ever so slightly widening the edges of the hole. The light flooding down into the tunnel suddenly darkened as objects could be seen moving around on top of their prison and then came the sweetest sound they could ever have heard—another human voice.

“Captain Hunter? Lieutenant O’Riordan at your service.” O’Riordan lay down on the ice and extended his arm into the hole. Alex could just reach the hand and he grasped it firmly.

“Glad to see you, O’Riordan. It’s getting a bit boring down here, mind if we come up?”

As Alex finished speaking a petrifying screech sounded from all around him. It finished with a grinding squeal as though rocks were being moved and pulverised under their feet.

“What the hell was that?” O’Riordan’s face, peering down into the hole, had paled and he quickly withdrew his arm.

“Just a little motivation, Lieutenant. We need to leave, right now.”