Immediately, it began fizzling and bubbling, which was only natural given that it was pure fluorosulfuric acid, the surface of which ascended rapidly as flesh and bone was fed downward into it. But they’d made the necessary calculations when filling the drum in the first place, and the girl was fully submerged before the acid reached its rim. Yellow Suit stepped back as the fumes began to thicken.

Satisfied, White Suit returned to the game.

Yellow Suit collected a thick plastic rod leaning against the brickwork in the opposite corner, and then turned back to the foetal figure in the drum. Though already unrecognisable as human, there must be air trapped inside it, because it was proving buoyant, semi-melted parts of it having broken back through the frothing surface. Yellow Suit applied the rod, shoving everything back out of sight into the hellish, seething soup.

A couple of minutes later, the rod was dipped into the first of three buckets of cold water, swished about and then replaced in the corner.

Yellow Suit stood back, hands on hips, seemingly pleased.

‘Haven’t you got more work to do?’ White Suit wondered through the microphone link connecting their two masks.

Yellow Suit glanced around, puzzled. ‘Nothing else I can do now.’

‘Mr Hopkins, idiot.’

‘What?’ Yellow Suit sounded outraged. ‘I’ve got to do that on my own too?’

‘That’s what he said.’

‘I’ve already had to do the respray on my own.’

White Suit went back to the game. ‘It’s the price of stupidity.’

‘There was nothing stupid about what happened back there. My foot slipped, that’s all. That’s why she was able to get the better of me.’

‘I seem to remember we’ve already had this conversation.’

Grumbling bitterly, Yellow Suit moved three paces to the right of the two drums, tested one of the flagstones with a cautious foot and, when it rocked, squatted down, fitted gloved fingertips into the crack and, with muscles straining, lifted. The flag was heavy but had been chosen deliberately because it was slightly too small for the square hole they’d originally slotted it into, so it didn’t need to be worked loose. When it was upright, Yellow Suit walked it away on its corners and rested it against the wall. Underneath, there was a circular grid made of chunky steel. It was badly corroded, but it still felt as though it weighed a tonne as Yellow Suit crouched again, reached between the bars and, with teeth gritted, lifted it free. This, at least, could be rolled out of the way.

Below that, a cylindrical rock shaft fell four feet into rushing water. Sidling around the edge of it, Yellow Suit moved back to the second drum. ‘Come on … at least give me a hand with this. It’s dangerous.’