‘Understatements,’ said Kumar. But I wasn’t going to give him the satisfaction of asking what the rumours were.

‘We drop down and have a quick look and if we don’t find anything we come back.’

‘Smelling of roses,’ said Kumar.

My dad says that the Russians have a saying: a man can get used to hanging if he hangs long enough. Unfortunately, what is true of hanging is not true of the smell of the London sewers, which are truly indescribable. Let’s just say that it’s the sort of smell that follows you home, hangs around outside your door and tries to hack your voicemail. Kumar and I ended up stuffing tissue paper up our nostrils, but agreed that if we had to come down again more drastic action would be justified – like amputation.

Since it was my idea I got to go first. The, let’s call it water, was freezing and knee-deep so that it cascaded over the top of my wellies. Later I learnt from a flusher, them that make their living maintaining the sewers, that only an idiot climbs into the sewers wearing anything other than waist-high waders. In my defence there were plenty of other idiots underground that night.

The ceiling was just high enough that I could wade upright, although the top of my helmet scraped the brickwork. I pushed upstream against the surprisingly strong current and Kumar splashed down behind me.

‘Oh god,’ he said.

‘Yeah I know,’ I said. ‘Water’s cold.’

‘That’s because it’s snowmelt,’ said Kumar. ‘That’s why we’re wearing wetsuits.’

I heard a splash from up ahead and pointed my helmet light in that direction.

‘There’s somebody ahead,’ I said.

‘Kill your lights,’ said Kumar. So I did and he followed suit.

It went completely black. I became aware of the sullen wash of the filthy water against my knees, of random sloshing sounds and a really disgusting slurping sound from somewhere behind us.

‘I think they heard us,’ I whispered.

‘Or there’s nobody there,’ Kumar whispered back.

We waited while the cold seeped into our legs. I’m not claustrophobic. It’s just that my imagination won’t let me forget how much the stuff above my head weighs. And if I start thinking about my breathing I start thinking about how it doesn’t seem to be bringing in enough oxygen.