‘Baaz,’ I said. ‘I need to ask you something.’ He slurped his tea and shrugged. ‘Last night I told you to run. But you didn’t. You were waiting for me when I came out. And this …’ I looked around the room again, and gestured to the empty breakfast platter, choosing my words carefully, ‘this, hospitality. Don’t get me wrong. I’m very grateful, but …’

‘But I’m in deep shit, aren’t I?’

‘Yes, you are. And I’m not sure I can help you, the way you’ve helped me. There’s nothing, I mean, I can’t …’

‘You saved my life,’ Baaz cut me off. ‘That’s why I didn’t run. And this,’ he said, lifting up his cup of tea, ‘is the least I can do. And besides, my auntie would kill me if she thought I wasn’t showing my friends proper hospitality. She takes that very seriously.’ I nodded. Baaz knitted his eyebrows, as if concentrating on solving a puzzle. ‘I know those men did what they did to me because of you. But I was breaking the law by being down there in the first place. So maybe that was my just deserts.’

I had to laugh at that.

‘I don’t think even the Russians have the death penalty for trespassing. What were you doing down there, anyway?’

‘Mapping,’ he replied. ‘To be honest with you, it’s a bit of an obsession.’ He looked away from me, at the table, and began drumming the fingers of his left hand as he’d done in the tunnels. ‘You see, I can recall the routes very well.’ He tapped his temple with his index finger. ‘Bloody crowded, remember?’

‘Yeah, I remember.’

‘But there are no maps. Not proper ones. The last decent one was made in 2011; but there are so many floods and collapses, that it was out of date as soon as it was printed.’

‘So?’

‘So that’s my challenge. For my dissertation. To come up with an algorithm that works with a map of the catacombs across all the levels – just like a map on a phone finds you the safest, most efficient route between multiple points, and doesn’t simply tell you if the one you’ve found is correct. It’s basically a three-dimensional, subterranean, travelling salesman problem. Only I’m plotting between tunnel junctions, not cities.’

‘And that’s hard, is it? To solve, I mean.’

He looked at me as if I was an idiot.

‘OK. Imagine you are a travelling salesman and you want to visit a certain number of cities before returning to the place you started. But you only have a limited amount of fuel for your car, so you need to find the shortest route.’

‘OK.’

‘If, say, there were, seventy cities.’

‘Right.’

‘Well, then the number of routes you would have to consider is more than the total number of atoms in the universe.’