Suddenly he was free.

The shark had spat him out. He'd evidently hit a delicate spot like an eye or a nostril. The enormous creature raced upwards through the water, passing close to him and sending him flying back into the rock. For a moment it looked as though it was turning tail. Bohrmann tried feverishly to think of a way to use the situation to his advantage. He had no illusions about what would happen if he tried to reach the Heerema. He'd temporarily got rid of the shark, but he had only a few more seconds. Hastily he pulled the trackhound towards him and threw his arms round its slender form.

Under no circumstances was he going to let it go.

The shark disappeared into the darkness and reappeared a little further on, a blue shadow in the water.

Bohrmann glanced frantically at the flank.

He was back at the crevice!

Some distance away from him the powerful body of the hammerhead was cruising through the open water. Bohrmann pulled himself towards the crack. He could see the other two sharks fighting over Frost's remains beneath the scaffold. They were moving down through the water, out of the illuminated zone. Bohrmann wondered how long it would be before they finished with the mangled body and turned on him. Then he stopped wondering anything. In the twilight of the ocean the big shark banked at incredible speed and came towards him.

Bohrmann pushed himself inside the crack.

There wasn't much room. The exosuit and the oxygen tanks on his back got in the way, and he struggled to shove himself in. Arms clamped to his sides, he tried to push himself deeper into the crevice, but the shark was upon him.

The cartilage of the hammer hurtled into the rock and the giant fish flew backwards. Its head was too big for it to enter. It arced round so tightly that it seemed to be chasing its tail. It tried again.

Chunks of lava dislodged themselves in a cloud of sediment from the surrounding rock. Bohrmann squeezed his arms closer to his body. He had no idea how far back the crevice extended. The shark was rampaging, attacking the rock, sending sediment and splinters into the water. Inside, Bohrmann was enveloped in fog. The blue light of the scaffold disappeared.

'Dr Bohrmann?'

Van Maarten. His voice was faint.

'Bohrmann, for God's sake! Bohrmann, say something!'

I'm here.'

Van Maarten made a noise that might have been a sigh of relief. Bohrmann could barely hear him amid the din the shark was creating. Noises sounded completely different in the water, like a dull, hollow racket of overlapping vibrations. The attack ended abruptly. He was stuck in the crack, blinded by the black cloud of mud. He could only guess where the scaffold might be.

'I'm in a crack in the flank,' he said.

'We'll send some robots down for you,' said van Maarten, 'and two men. We've got more suits.'

'Forget it. The PODs don't work.'

'I know. We saw what happened to-' Van Maarten's voice failed him.

'We'll send the men right away. They've got harpoon guns with explosive charges and-'

'Harpoon guns? Now, there's a thing,' Bohrmann said caustically.

'Frost was convinced you wouldn't need them.'

'Evidently.'

Something rammed Bohrmann in the chest, pushing him deeper into the crevice. He was so surprised that he forgot to scream. In the dim light he saw the hammer. It had hit him vertically. The shark was trying to enter the crack on its side.

Why you clever little thing, he thought grimly. His heart was in his throat. I'm going to make you pay.

He rained blows on the hammer, careful not to let go of the hound. He could vaguely see its jaws opening and closing. The rectangular head was beating up and down, but Bohrmann was out of reach of its jaws. Its eye was rolling. Bohrmann raised one of the manipulators and let the console slam down on top of it.

The shark flinched.

It's not going to be able to get itself out of here, Bohrmann realised. He channelled his strength into pressing the trackhound against the shark's skull. Surely the creature couldn't be jammed. How much power did the jelly have over it? It was obviously controlling its behaviour, but could it teach it to swim backwards?

Evidently it could. The hammer withdrew from the crack.

Bohrmann waited.

Something shot out of the cloud. A hammer came at him horizontally. One of the smaller sharks. Its head crashed into the domed visor of his helmet. Its jaws opened. Rows of teeth scraped against the Plexiglas. The shark's body obscured the light to such an extent that Bohrmann could barely see, but what he could see was enough. He tried to push himself further inside the crack and suddenly the walls of the crevice seemed to give way. He toppled backwards into nothing.

Pitch blackness.

The left manipulator moved erratically over the console. The switch for the trackhound's floodlight was just above the homing button. He'd had it a moment ago…

There!

The floodlight lit up. The wandering shaft of light revealed that the back of the crevice had widened into a spacious cave. He shone the beam at the opening and saw the head of the shark. The hammer was shaking back and forth but the shark didn't advance.

It was stuck.

Bohrmann raised his arm and showered blows on the box-like head. The shark had to be at least half-way into the cave. Suddenly he realised that it wasn't a good idea to wound the shark enough to make it bleed. Instead he used all his weight to push against it, but in the water it wasn't nearly enough. He pushed off and hurled himself against the twitching head, banging into it with his chest, shoulders and arms until the shark gradually retreated. The beam from the trackhound wandered all over the place, illuminating the pink gullet and flapping gills.

I don't care how you get out of here, thought Bohrmann. But I want you out now. This is my cave, so piss off!'

'Piss off!'

'Dr Bohrmann?'

The shark disappeared.

Bohrmann slumped down. His arms trembled. Suddenly he felt overwhelmed with exhaustion and sank to his knees.

'Dr Bohrmann?'

'I don't need you bugging me, van Maarten.' He coughed. 'Do something to get me out of here.'

'We'll send down the robots and the men right away.'

'Why robots?'

'We're sending down anything that might scare the sharks or distract them.'

'They're not sharks. They only look like sharks. They can recognise a robot – and they know exactly what we're trying to do.'

'The sharks know?'

Frost evidently hadn't told van Maarten the whole story.

'That's right. They're no more sharks than the whales are whales. Something's controlling them. The men should be on their guard.' He had to cough again, this time more loudly. 'I can't see a bloody thing in this cave. What's going on out there?'

For a moment van Maarten was silent. Then he said, 'Oh, God…'

'Talk to me!'

'There's more of them – dozens, hundreds! They're smashing up the floodlights.'

Of course they are, thought Bohrmann. That's the whole point.

They're trying to stop us cleaning up the worms. That's what this is about.

'Then forget it.'

I'm sorry?'

'I said forget the rescue operation, van Maarten.'

There was so much noise inside Bohrmann's helmet that he had to get van Maarten to repeat his answer a second time: 'But the men are ready.'

'Tell them that intelligent predators are lying in wait for them. The sharks are intelligent. The stuff in their heads is intelligent. You're not going to achieve anything with two divers and a decoy. Think of something else. Like you said, I've got enough oxygen for two days.'

Van Maarten hesitated. 'OK. We'll keep an eye on things. Maybe the sharks will disperse in the next few hours. Do you think you're safe for the moment?'

'How the hell do I know? I'm safe from ordinary sharks, but these guys are unbelievably resourceful.'