'I know what you mean.'

Her breath was coming a little fast, and she tried to slow it, talking about the tides for a moment and the state of the sea, until she finally said, and had not been willing to say, 'All right, when you're ready.'

By the brass chronometer in the cabin of the tug it was eight in the evening when we anchored over deep water, a few minutes after eight, though time had lost its meaning now, and there was no hurry.

'How far are we?' I asked her.

'Two sea miles, give or take a bit. That's what you said you wanted.'

'Are we under their radar?'

'Yes. But we're only a blob. They don't know what vessel she is.'

The sea was dead calm, and the lights between here and the coast were motionless. The moon hung among hazy stars, and you would have said it was a night for magic to be made across this vast unfathomable stage, a night for sorcery, its reaches peopled by warlocks, witches and diabolists, casting their spells and conjuring phantoms from the very air. I told you, my good friend, that by nightfall I would come to know that I was mad, and here was the night, and here this madman's tale.

Here too of course the appalling urgency to turn back and gain the shore and find a telephone and offer myself to be led like a lamb to London.

'Did you eat anything?' she asked me.

'Yes.'

'Not too much.'

'No. Some sardines.'

Time to go to the loo.'

When I came back she helped me on with the wet-suit, and I asked her, 'All right, what am I up against?'

I heard her take a breath. 'Bad news first. They feed by night, and actively. There are a lot more rods than cones in the retinae, so they can see quite well in dim light. The moon isn't a help, though you'll need it to see what you're doing.'

I pulled the front zip and began strapping the ankles. She helped me, crouching at my feet, her hands quick and deft. 'The things that attract them are light, noise and rapid movement. I suppose that's true for most creatures, it's nothing special. But watch out if you see garbage being thrown overboard, and keep well away from it. They sometimes move about in packs, as you saw yesterday, but ninety per cent of attacks are made by a single shark. The attack's usually direct, straight on, without any close passes beforehand.' She straightened up and began helping me with the gear and the floats. 'Statistically, which is really all I'm talking about, only a third of the vie – of the people attacked have reported seeing the shark. It's usually what we call a blind hit, before you can see anything.' She stopped talking for a minute; I suppose she was having trouble with a buckle or something.

That's all I need to know,' I said. 'You've -'

They're also attracted to fish moving in a shoal. If you see a shoal, steer clear of it or try to swim towards it to turn it away. Most of the strikes are made at the extended arms and legs; try to remember to swim with your flippers almost together; just paddling slowly, with your arms close to your sides.

I wish to God -' she said and broke off and for the rest of the time she managed to sound almost normal, with her voice no more than subdued.

'What's the best weapon?'

'I'll come to that,' she said. 'These things have got large olfactory sacs, and their sense of smell is acute. A test they made at Lerner's showed that a shark can detect one part of tuna juice in twenty-five million parts of sea-water. When they smell anything that interests them, they turn upstream and home in on it. So if you feel any current running and you see a shark upstream of you, you're in better shape as far as your scent is concerned. I know you probably won't have to use any of this but if you do run into problems it's going to give you an edge.' She was standing in front of me now, fastening the last strap of the scuba harness, her eyes watching me in the light from the binnacle, the green pupils iridescent, darker than I'd seen them before, more concentrated, and I had the passing thought that she was looking at me for what might be the last time, but if that kind of thing was in her mind she would be wrong, she would of course be wrong.

'Does everything feel okay?'

I shrugged the harness a bit higher and she took up the slack on the buckles. 'Okay now?'

'Fine.'

She turned away and got a metal cylinder from the cabin, black-painted with a fire-extinguisher type lever. That was all the bad news, as I said. This is the only good news we've got. It's a concentrate from the Moses sole fish, gives out a milky fluid, but the toxicity's only potent enough if it's released into the shark's mouth.' She buckled it to the left side of the harness at the hip. 'Don't forget you've got it, for God's sake. Everything comfy?'

I said yes and walked to the rail and she held the gear steady while I climbed over and turned my back to the sea and looked up at her as she offered me the unexpected miracle of a quick, flashing smile and I let go and the cylinders hit the surface and spread bubbles around me like a veil of white lace as I turned over and began swimming.

It was huge, a long shadow lying under the surface.

I'd heard someone say it was two thousand tons, the size of a destroyer. It looked even bigger than that, its outlines etched by the play of moonlight through the water, broken by a shoal of fish swarming near the twin screws aft, flashing as they turned, darkening and flashing, their quickness mesmerising.

There were sounds here, muffled but not distant, the sound of generators and voices and music, so faint sometimes that I believed that silence had come, then getting louder as the current swirled and I rose through the water, breaking the surface under the dark slope of the keel. There was no music now; it hadn't been a party on deck or anything; I think it had come from radios in the crew's quarters, aft, where I'd approached the target, the motor-yacht Contessa.

I began work straight away, and fixed the first one a foot above the surface on the starboard side. The magnet was strong, and made a sudden ringing sound as its field pulled it to the hull with the force of a hammer blow.

I hadn't been ready for that. I didn't like it. A fish, even a big fish, moving at speed and turning, hitting the hull obliquely, wouldn't make a sound like that. I think it would have been heard, inboard, I think it would have been heard by people in the well of the ship.

I used the flippers to drive me below again so that I could take sightings. I didn't feel comfortable with the lower half of my body dangling below-surface. Looking down the length of the hull I could see the shoal again, a swarm of two or three hundred small fish, flashing silver as they turned and turned again with a speed that gave them the semblance of an illusion.

They're also attracted to fish moving in a shoal. If you see a shoal, steer clear of it or try and swim towards it to turn it away.

I was all right here: they were as distant as the length of the ship. There was no other movement anywhere, except for bubbles rising from vegetation on the sea bed. The anchor chain hung in the water not far off, under the bows, a rope of black pearls in the filtered light of the moon. In the other direction the twin screws bloomed like dark flowers, their rounded petals silvered at the tips. The moon was above the port beam, so that one half of the hull was dark, the other barely visible, lit from the surface as brightly as the sea itself and merging with it.

I moved slowly to the other beam, and spread one hand against the painted metal, palm towards me, and laid the next unit over it; but the magnet was stronger than I'd thought and it was a job to pull my hand free, and when I did there was still a slight hammering sound as the unit met the ship's plate. I would have to do better than that.