They all passed under the choke and Alex yelled for them to keep moving as fast as they could and to stay close to the walls to avoid any falling debris. He turned and calibrated his rifle to deliver fist-sized compressed air punches to the weakest point in the choke and depressed the trigger. A constant stream of air projectiles was focused on where the choke met the wall. The high-velocity compressed air hit the rock face with the power of a pneumatic sledgehammer. In just under three seconds, Alex managed to do what the rocks had tried to do in their original rush to the cave floor tens of thousands of years before; they came boiling down with a thunderous crash. He threw his rifle over his shoulder into the backpack strapping and leaped forwards, bouncing nimbly over and through the falling stones. Tons of rock and debris collapsed around him. The impact, when it came, was like being hit by a car. The centre of his back was punched by a rock the size of a bowling ball travelling at about forty miles per hour. The breaking sound from his back was lost in the crash of the rock slide, but Alex was knocked to the ground.

He lay on the cave floor waiting for the dust to clear. His senses told him there was no more airflow and that most of the debris had fallen, totally blocking the narrow cave. He doubted whether the creature could bring enough power to bear with just a single tentacle to force a path through. They were safe for the moment.

Alex sucked in a deep breath and spat out some dust. He slowly got to his feet and flexed his upper body; he hurt and something crunched at his back. He pulled off his pack and pulled his rifle free. The once sleek M98 was a crumpled mess. It had been made from thermopressed polycarbonate—stronger than steel but as light as plastic. The impact had shattered the casing. Better you than me, he thought, and dropped the crushed gun at his feet.

Aimee was waiting just behind a large boulder with her canteen for him to take a much needed swallow.

“Well, looks like we can rule out doubling back from now on. Let’s take ten minutes.” They all fell to the floor. Now that the adrenaline was subsiding in their systems, exhaustion was taking over. Muscles screamed, feet throbbed and backs ached. Alex lay down flat and closed his eyes. He gave them ten minutes, so he would sleep for exactly eight before rousing them to continue the next stage of their journey back to the light.

“It’s a door.” Matt was dwarfed by the structure.