'Hey!' The console slipped away from his articulated grasper. 'What in God's name-'

A third body shot over the scaffold, appearing from nowhere. It sped over the line of floodlights with eerie elegance. A tall, dark dorsal fin, and a hammer-shaped head.

'Stan!' screamed Bohrmann.

The latest arrival was enormous, much bigger than the other two. Its hammer lifted upwards as it opened its jaws and grabbed Frost's right arm and tugged.

'Shit!' he yelled. 'You evil bastard, let go of me, you-'

The hammerhead wrenched its huge rectangular head from side to side, using its tail to steady itself It had to be six or seven metres long. Frost was shaken like a leaf His suited arm had disappeared up to the shoulder into the shark's gullet. 'Beat it!' he screamed.

'For God's sake, Stan,' yelled van Maarten, 'punch it in the gills. Try to hit its eyes.'

Of course, thought Bohrmann. They're watching us. They can see everything.

Bohrmann had sometimes wondered what it would be like to encounter a shark, be attacked, or see it go for someone else. He was neither particularly brave nor especially fearful. Some would have deemed him an adventurer. He might have described himself as a man who was not afraid to take risks, but who didn't go looking for them. Now, faced with the huge predator, it didn't matter how he or anyone else had judged him in the past.

Bohrmann didn't flee from the shark. He swam towards it.

One of the smaller creatures approached him from the side. Its eyes twitched and its jaws jerked open. It evidently required great effort for it to swim into the electric field. It accelerated and rammed into Bohrmann.

He was thrown to one side and fell through the water towards the scaffold. All he could think about was not letting go of the console. Come what may, he had to hold on to it. Without its homing program, he'd be doomed to swim around blindly in the dark until his oxygen ran out.

Assuming he lasted that long.

A sudden surge of pressure caught him and pushed him downwards. The tail of the big shark thrashed above his head. Bohrmann tried to regain control of his movements, and saw the two smaller sharks swim towards him in formation, jaws snapping. They were so close to the scaffold that their natural colours were illuminated in the blue. Bronze skin stretched over their backs towards their white bellies. Their gums and gullets had the orange-pink glow of freshly filleted salmon. Distinctive triangular daggers lined their upper jaws, with pointed teeth stacked below – five rows like sharpened steel, positioned one after the other, ready to tear into anything that came within their reach.

'G-a-i-r-h-a-r-d!' screamed Frost.

Squinting into the halogen lights, Bohrmann watched Frost raise his free arm and rain blows on the head of the shark. Then, in a single violent shake of its head, the shark ripped the arm from the exosuit and cast it aside. Fat oxygen bubbles escaped from the tear. The jaws opened and snapped shut on Frost's unprotected arm, biting it off at the shoulder joint.

A cloud of blood and bubbles billowed darkly in the water. So much blood. The sweeping motion of the shark's tail dispersed it. There were no words to be heard in Frost's screams, just unarticulated high-pitched sounds, then a gurgle as the seawater shot into his suit and filled it. The screams stopped. The smaller sharks lost interest in Bohrmann. Whatever was controlling their minds couldn't stop their natural instincts coming briefly to the fore. They rushed into the turbulent water and dragged Frost about as they tried to bite through his suit.

Amid the static van Maarten was screaming too.

Bohrmann was paralysed with shock, yet at the same time part of his brain was crystal clear. It was telling him that he shouldn't rely on the creatures to follow their instincts. Their power and hunger were being manipulated. This wasn't about feeding. Temporarily their instincts had got the better of them, but the substance inside their heads was interested in one thing only: killing the human intruders.

He had to get back to the flank.

His left-hand manipulator reached towards the keypad on the console. If he made a mistake, he would activate the homing program and launch himself towards the Heerema, which – now that the POD could no longer defend him – would inevitably cost him his life. Somehow he hit the right button. The propeller began to whir, and he manoeuvred the joystick so that the hound was pulling him away from the scaffold and towards the lava flank. He could feel the acceleration. On the way down the robot had seemed speedy and dynamic, but now it trundled along at an interminable crawl.

Bohrmann kicked his fins and glided through the water towards the terrace. There wasn't much he could do in a situation like this, but one of the rules of diving stated that rocks afforded protection. Bohrmann progressed towards the wall of lava. As he reached it, he turned and stared up at the scaffold. Fins and tails thrashed in the dissipating cloud of blood, creating a maelstrom of bubbles. Sections of Frost's suit sank through the water. It was a harrowing sight, but what truly horrified him wasn't the bloodbath; it was that only two of the sharks were involved.

The big shark was missing.

Numbing fear took hold of him. He turned off the propeller and looked around.

The big shark shot out of the cloud of sediment with its mouth wide open. It was coming at him with breathtaking speed. This time Bohrmann's mind shut down. He still hadn't worked out whether or not he should switch on the trackhound when the wedge-shaped head rammed into him, flinging him backwards into the flank. He hit the lava with a dull crunch. The shark swam past, then returned at the speed of a racing car. Bohrmann screamed. Now there was nothing but an abyss of jaws and teeth, as the gaping mouth took his entire left side, from shoulder to hip.

Well, that's that, then, he thought.

The shark shot over the terrace, pushing Bohrmann's body through the water. There were rustling and droning sounds in his headphones. The shark's teeth grated against the titanium shell. Its head swung back and forth, banging Bohrmann's helmet against the lava, and scraping it along the flank. The world was spinning. The titanium alloy was tough enough to withstand the battering for a while longer but inside the suit Bohrmann's head was banging mercilessly from side to side. He couldn't see or hear. His fate was sealed. He was going to he sawn apart and ripped to shreds. His life was worth less than the air in his lungs.

It was the helplessness that enraged him.

He was still breathing, wasn't he?

Then he could fight back!

The straight edge of the hammer stretched out above him. The head's width was equivalent to a quarter of the shark's total length, which meant that he could see only the hammer's edge: no eyes or nostrils. He started to hit it with the console. The shark swam on, heading towards the edge of the light where Bohrmann had waited with Frost for the charge to explode. Once they were in the pitch-black water, he wouldn't even be able to see the shark.

They had to stay in the light.

Bohrmann exploded with rage. Trapped inside the shark's jaws, his left arm jerked up and pounded its palate. It was lucky that the shark had seized his side and not just an arm or a leg – otherwise he would have met Frost's fate. There were no weak points like articulated joints in the metal shell protecting his torso. It was too big and solid, even for the teeth of a predator like this. The shark seemed to realise that too. It shook its head more vigorously, until Bohrmann was on the verge of blacking out. He'd probably broken several ribs already, but the more the shark shook him, the angrier he became. He bent his right arm, reaching up towards the end of the hammer, and smashed the console against it-