I drop down into the cold dark water into what is probably a comedy crouch, my arms held out in front of me, and head right. From what I’ve seen of the repeat pattern of architecture here, and from where his voice is coming from, Dad is in the space – cavity? – hole? – beyond that final eyelet arch. There’s a foot-high threshold to climb over and I clutch around for it. This would be comical if it wasn’t so scary. If I wasn’t such an out-and-out wuss. I used to have a lamp on in my bedroom at night, for goodness’ sake! I’m clutching at the air. I’m clutching at nothing. There. There’s the lower wall of the eyelet. I clamber over it, like a very ungainly otter, and down the other side.

‘I’m here.’

‘So am I, Dad.’ I clamber forward and reach out and I can feel his arm. His shoulder. The soft material of his Fred Perry T-shirt. ‘Thank God. I found you.’

‘You found me, Roo,’ says Dad and his voice suddenly sounds very small and very Italian.

‘What on earth happened? And where’s Kemp?’ I ask again.

‘He’s down here,’ says Dad. ‘But I’m not sure where.’ He clutches for both my hands now and I hold his tight. They are a warm, familiar relief. ‘I got bored, waiting up there. Kemp came down on his own, as we planned, and I sat up there, like a lemon, just waiting. Haven’t I just sat waiting most of my life? After I’d waited for ages, I decided to come down the ladder and stand in here for a while, feel the acoustics, touch the walls. I knew there were nine rungs. I got to the top of the steps and I called for Kemp but heard nothing so I went down the steps.’

‘You went down the steps? On your own?’ My voice echoes in the pitch dark. It bounces off brickwork and still water.

‘Well, yes. I can walk down steps. I thought I’d be OK … Anyway, when I got to the bottom I called again and there was no answer. I started to worry because I knew Kemp was down here somewhere. So I rolled up my trouser legs and I walked two or three arches.’

‘For God’s sake, Dad, this isn’t like paddling on the beach at Southend!’ I exclaim. ‘It’s dangerous down here!’

‘I was fine, for a while. I’ve been down here before, remember? I’ve read about it since. I know the layout. But I had to look for him. If he wasn’t answering, then he had to be in trouble. I was calling him all the time. Nine arches in, I finally heard something. A moan. So I came looking.’

‘And you ended up all the way down here.’