In the next half-hour or so I fixed four more of them, using a fabric strap as a buffer to deaden the sound, and then duck-dived to take another sighting below, and saw the shark.

It was half the ship's length away and looked motionless, a ten or twelve foot grey cylinder, flattened a little horizontally. It was just below the surface, its profile silvered by the moon and not easy to see, except for its size. Then it began moving, at first across the beam of the ship and then turning to stand off again, nearer me but not close yet. It was pointed now towards the stern, and did nothing for a while; then the long tail-fin moved suddenly and it was streaking the length of the hull and hit the centre of the shoal before the fish had time to scatter. It looked like a big window being smashed, with the bits of glass exploding from the centre.

I was about midship, and needed to move aft. The shark had turned and was facing towards me, but at a fair distance. What was left of the shoal had regrouped and was shimmering in the water near the hull again, apparently unable to learn.

Itching on the skin, the nerves shaken and sweat springing, not unexpected. I'd done most of the work down here in comfort, free of any concern except for the noise of the magnets jamming home, but now things had changed.

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.

It was time to remember the other things she'd told me, and I kept my arms close to my sides and my legs together, drifting closer to the hull. There was so little movement of the water against my hands that I wasn't sure there was a current at all; but if there was, I was downstream of that bloody thing and it couldn't smell me. But of course it could see me: she'd said they could see well enough in dim light, and the moon was bright enough through the clear water to define the shark's dorsal fin even at this distance. It could see me very easily.

You've placed six of those things. Now get out.

We need eight. That man Parks recommended eight.

Six are good enough. For God's sake get out while you can.

Panic will get us nowhere. I shall stay exactly where I am.

But it wasn't easy. It was not easy, my good friend, to stare at that hideous two-ton killing machine while it stared me back. It had kept still like this, just like this, before it had suddenly shot forward and hit that shoal like a missile.

For the sake of Jesus Christ get out, get out, get out.

Sweat crawling on the skin under the wet-suit, itching, making me want to move, to pinch the flesh through the rubber, the only way to scratch. But let us be reasonable; nothing much has changed, when you stop and think. I knew this was dangerous, and I knew Ferris would have tried to stop me if I'd told him what I meant to do, and I knew that by the end of this long day I would come to realise that I was mad, and that when she had given me that wonderful smile, when she had mustered all the courage she had needed just to do it, to give me that flashing beatific smile, I knew that she hadn't thought much of my chances, that she was in all likelihood looking upon this vain and ambitious madman for the last time, and had managed to bring herself to offering him everything of life she could, the gentle valedictum, the grace of her womanhood. I knew those things.

But somewhere along the line, as they say, I'd been lulled into thinking it was going to be cushy down here after all, because they wouldn't come, the sharks, wouldn't seek me out, wouldn't decide to make of this impudent clown a snatched meal, the jaws coming open as the great body turned with the tail driving it towards the kill, the jaws locking shut on impact and the flesh becoming shreds, the bones -

Out, get out for Christ's -

Yes, I'm afraid I got carried away a little, didn't I, and if you weren't quite so shit-scared I wouldn't have to suffer your pusillanimous bloody whining, I'd have a better chance to think.

Think.

Move very slowly, paddle with your flippers, arms to the sides, move towards the stern, towards that great grey fish with its tiny eyes, keep close to the hull, just underneath it, part of the ship, just a piece of equipment, nothing alive, nothing of flesh and blood, the jaws coming wide open as it – steady, lad, we came here to do this and we are going to do it, the sweat crawling, ignore it, ignore the itching, driving me crazy, ignore.

Then it moved and grew enormous as it drove past me and hit the shoal again and the fragments scattered and I held still with the breath blocked in my throat and my senses numbed, held still, a piece of equipment, nothing alive like the little fish over there, some of them crushed but slipping out of that cavernous mouth again, floating to the surface, awkward-looking, their blood trailing in the light of the moon.

Door banged somewhere, some kind of door, the clang of metal, and then the light was dappled with movement as things began drifting down, touched with silver and sending out small bubbles, some kind of things I didn't know what, my mind was too occupied with the shark over there, nosing through the water while the things went on drifting down, surrounding me, an apple-core moving against the face-mask, an apple-core, mother of God, the things that attract them are light, noise and rapid movement, but watch out if you see garbage being thrown overboard and keep well away from it.

Egg-shells, a chicken carcass, potato peelings, drifting around me and I started moving forwards, keeping horizontal and just below the hull but too close and the air-bottles banged against the plates and I froze and waited for the shock to pass and then put my head down and went lower, fanning with the flippers, not looking to see where the shark was because it would mean turning and I didn't want to turn, to move more than I had to. It was somewhere behind me now, the big grey fish, but I didn't know how far away. Only a third of the people attacked have reported seeing the shark. It's usually what we call a blind hit.

But suddenly it was in front of me, small in the distance, and I hadn't seen it go by. It must -

Two. Two sharks now.

A vessel of this size, I suppose, would attract attention at night, at feeding time. The memory cells inside those tiny brains would automatically steer them towards garbage. With this amount of light -

Three. Four.

They were zeroing in from the featureless expanse of water wherever I looked, and the big one behind me went past at a distance of a dozen feet as I floated just beneath the hull, a part of the equipment. The thing hadn't accelerated this time; it knew that garbage doesn't scatter when attacked. Two others -three others came in much faster, competitively, and went for the debris, brushing one another, making tight turns with their jaws wide, taking in what they could get.

The metal door clanged again and more garbage mottled the surface, dark at first and then catching the light as it sank, and the four sharks – five – became excited, dog-fighting their way through the debris, and one of them broke off and brushed against the propellers, jaws open, ready to attack anything, any shape at all, then it turned very fast and came for me head-on and I held still for as long as I could and then I was looking directly into the gape of the jaws and brought the cylinder up and squeezed the lever and felt the coarse hide graze past my shoulder as the toxic fluid clouded the water like milk and the tail fin hit me and the air-bottles rang against the hull.

Sake of Jesus Christ get out, get away.

Things not good, a degree of concussion, it had been a blow to the head, but I was aware of what was going on, though not terribly interested, blood in the water now ahead of me, 'a blossoming of crimson, perhaps one of them had gone for another, becoming frenzied, I didn't actually care whether -