From down below, another explosion echoed towards them. There were tons of munitions down there, Porter realised, and one by one, the flames were reaching the stockpiles of weaponry, igniting vicious fireballs. The ground beneath them shook, and a pile of dusty debris scattered loose from the ceiling, showering them with fragments of rock and ore. Porter started to pick up the pace, jogging alongside Hassad. As they moved away from the shaft they had climbed up, they lost all contact with the light. Within seconds, they were shrouded in darkness. It was too dangerous to run: they could see nothing, and the path was littered with rocks and pits they could easily trip over. Hassad was hugging the wall, feeling his way forwards with his hands, and Porter was following on behind. The metres were covered painfully slowly. A couple of times Porter could feel himself starting to fall as his feet collided with a rock – and a fall might easily finish Katie off in her weakened condition – but he just about managed to recapture his balance in time.

Eventually they reached a doorway. A light was shining underneath it: the glow of burning flames. Even standing ten feet from it, Porter could feel an intense heat from the other side. ‘It’s on fire,’ he said, looking towards Hassad. ‘There must be another way.’

Hassad shook his head. ‘Back there, the mine just goes deeper and deeper underground.’

‘It’s burning up over there.’

‘It’s the only way,’ Hassad snapped. ‘Otherwise, we die down here.’

He took a couple of paces forwards. The exit was made from an old metal fame, with a wooden door inside it. There was a padlock, but it didn’t look very strong. Hassad pushed his shoulder against it, then stepped back a couple of metres and charged the door. It snapped free on impact. The padlock broke away, and the rusty hinges collapsed, sending the door crashing to the ground. Porter followed Hassad through. Katie was still clinging to his back, wheezing heavily as a fresh blast of smoke hit them in the face. He was looking into a long thin room, maybe thirty metres deep by five across. A burst of heat hit him in the face. At the far end, he could see the lift that led down to the level below, but it was smashed to pieces and twisted beyond recognition. Flames were licking up from that level, creating the waves of heat that had hit him in the face. Behind the lift shaft, there was a thick, broad channel, reaching down into the mine, and above up to the night sky. Porter reckoned that must have been where the bunker-busting missile came in: its hardened, titanium nose would have pierced the ground, and then bored its way easily through the levels of the mine, before exploding viciously deep underground. Bunker-busters were designed to take out the reinforced concrete layers that protected nuclear or biological weapons: it was no surprise that it had shredded the mine like a carving knife through a sandwich.