Sergeant ‘Flea’ Marley, the head of Avon and Somerset ’s underwater search unit, slid into quarry number eight at just after four on a clear May afternoon. She wasn’t thinking about secret entrances. She wasn’t looking for holes in the wall. She was thinking about a woman who’d been missing for three days. The woman’s name was Lucy Mahoney, and the professionals on the surface believed her corpse might be down here, somewhere in this vast expanse of water, curled in the weeds on one of these ledges.

Flea descended to ten metres, wiggling her jaw from side to side to equalize the pressure in her ears. At this depth the water was an eerie, almost petrol blue – just the faintest milky limestone dust hanging where her fins had stirred it up. Perfect. Usually the water she dived was ‘nil vis’ – like swimming through soup, everything having to be done by touch alone – but down here she could see at least three metres ahead. She moved away from her entry point, handholding herself along the quarry wall until the pressure on her lifeline was constant. She could see every detail, every wafting plant, every quarried boulder on the floor. Every place a body might have come to rest.

‘Sarge?’ PC Wellard, her surface attendant, spoke into the comms mike. His voice came into her ear as if he was standing right next to her. ‘See anything?’

‘Yes,’ she murmured. ‘Into the future.’

‘Eh?’

‘I can see into the future, Wellard. I see me coming out of here in an hour freezing cold. I see disappointment on everyone’s face that I’m empty-handed.’

‘How come?’