Not many countries have access to bunker-busting technology, thought Porter.

But the British do. And the Israelis.

‘This way, quick,’ yelled Hassad. ‘We haven’t much time.’

Porter now saw he was pointing towards a corridor that led away from the main room.

Porter glanced around. The area near the lift where they had come in last night was clearly impassable. The missile had left a pile of burning debris. Even the rock looked redhot: touch it and it would set fire to you instantly. The timbers along the room had ignited and burnt, and the floor was now a smouldering mess of ash and embers. There was no way they could cross it. But the corridor Hassad was pointing to looked OK. If I can trust him, thought Porter.

Checking that Katie was secure on his back, Porter started to run across the open space towards the corridor. He could feel great waves of raw heat from the lift, and a couple more explosions rocked through the mine in quick succession. The ash and embers on the floor were burning into his feet: running as fast as he could, he could feel it singeing the soles of his trainers. The ground was vibrating all around him, and the walls were shaking. Another explosion, and then a vicious fountain of sparks shot upwards from the lift shaft like a display of fireworks. This mine can’t take much more punishment, Porter reflected grimly. Any moment now, the whole place is going to blow.

Hassad had already turned into the corridor. Porter ran after him. The tunnel was long and thin, twisting up and down through the rock. It was held in place by a series of wooden timbers, many of which were already burning. The heat was searing, like stepping straight into a microwave. Porter could feel the sweat dripping off his skin, and on his back, Katie was coughing viciously, the flannel across her face doing little to control the heavy fumes from the fires all around them.

They covered ten, twenty, then thirty metres. Porter wasn’t sure how much more he could take. The temperature was rising all the time. His head was throbbing and his vision was starting to blur: beyond a certain point, he knew that men just dropped from heat exhaustion.

‘We can’t make it much further,’ he shouted to Hassad.

‘Just another ten metres.’

Porter pressed on. He could see the tunnel opening up ahead. There was a wooden structure up in front of them, with flames already licking around it, and beyond that a rickety wooden staircase that had not yet caught fire. Behind him, the heat was growing more intense by the second, and he’d already heard the sound of rocks crashing to the ground. As they burnt, the timbers could no longer support the weight of the roof. It was groaning, threatening to collapse at any moment. Ignoring Katie’s cough, he pushed on harder, picking up the pace. There might only be seconds left before the whole place collapses, he told himself. Each step could well be the last one.