For a weak one, it could be fatal.

‘Try not to breathe too deeply,’ he muttered.

Porter stepped out into the corridor, pressing Katie’s face into his chest to try and keep her mouth away from the smoke. The fumes were so heavy, and the light so poor, it was impossible to see more than a few metres ahead. From somewhere, Porter heard a shout, telling him that not everyone was dead. Not yet anyway. Hassad was already striding towards the meeting point. Porter plunged forward, ignoring the carnage all around him. As he stood alongside Hassad at the centre of the mine, there were at least five bodies strewn across the ground, all of them burnt beyond recognition.

‘How the hell do we get out of here?’ Porter shouted, though his voice was muffled by the wet cloth strapped across his mouth.

‘All the exits are blocked,’ said Hassad.

Porter glanced around. The timbers the miners had wedged into place to hold up the roof years ago had been incinerated in the blast. Some were still burning and the roof was starting to sag badly: Porter reckoned it might not be long before the whole thing came down, bringing a couple of hundred tons of rock collapsing onto their heads. The staircase that led down from the lift into the meeting point had been totally destroyed. Flames were still licking upwards where the timber frame of the stairs had once been. It was totally impassable.

‘We’ll take the back way,’ said Hassad.

Along one of the tunnels, Porter could hear an explosion. There was suddenly a blinding flash of light that cut through the fumes and illuminated the meeting point. Porter shielded his face as he felt a blast of heat wash over him. He gripped tighter onto Katie, and tried to follow Hassad. He’d already started marching towards a tunnel, motioning to Porter to follow him. Porter stepped forwards. The fire was gathering pace behind him, and he could hear a scream as it collided with one of the men trying to escape, frying him in an instant. Keeping hold of Katie, he followed Hassad into a narrow channel cut into the rock. There were a couple of bodies at its entrance, but Porter just stepped over them and pressed on. Behind him he could hear the sound of another roof collapsing somewhere, and more screams.

Hassad was twisting his way through a tunnel that was narrowing all the time. The light from the flames licking up around the meeting point had illuminated the first few metres, but it was fading fast, and within a few seconds Porter was struggling to keep track of Hassad through the darkness. A wall loomed up ahead of them, but to the right-hand side, there was a crack that appeared to lead upwards. It was part of an old ventilation shaft that must have been built years ago. ‘Here,’ said Hassad. ‘This will take us to the next level. Where the lift is.’