'It's working!' yelled Frost.

'You guys are the best,' van Maarten said drily.

'Tell me something I don't know,' agreed Frost. 'Now switch it off before it eats us. We'll check out the site again, and then we're coming up.'

The tube lifted a little, then its mouth drooped and it dangled lifelessly amid the light. Bohrmann set off. He glanced over at the scaffold, then back again. Something didn't look right, but he couldn't put his finger on it.

'A shady business,' said Frost, jerking his head towards the gloomy cloud. 'Go ahead, Gairhard. You'll be able to make more sense of it than I can.'

Bohrmann switched on his trackhound's floodlight. Then he thought better of it and switched it off.

Was he seeing things?

He glanced at the scaffold again. This time his eyes lingered. It seemed that the floodlights were more powerful than before, but that was impossible: they'd been on full beam throughout the operation.

But the glow wasn't coming from the floodlights. It was coming from the blue aura. It was getting bigger.

'Do you see that?' Bohrmann jerked his arm towards the scaffold. Frost's eyes followed the movement.

'I can't see- My God.'

'The light,' said Bohrmann. 'The blue glow.'

'By Ariel and Uriel,' whispered Frost, 'you're right. It's spreading.'

A blue-violet halo had formed round the scaffold. Distances were hard to judge under water, particularly since the refractive-index made everything look a quarter closer and a third larger than it was – but the source of the blue glow was clearly a good deal further away than the lighting unit. Although the glare of the halogen lamps was shining into his eyes, Bohrmann was almost certain he'd seen flashes. Then the blue paled, the light faded and went out.

'I don't like the look of this,' said Bohrmann. 'We should go back.'

Frost didn't answer. He was still staring at the scaffold.

'Stan? Are you listening to me? We should-'

'Don't do anything hasty,' Frost said slowly. 'We've got company.'

He pointed to the top of the scaffold. Two long shadows were patrolling the length of the frame. Blue bellies flashed in the light. Then they were gone.

'What was that?'

'Don't panic, kiddo. Turn on your POD.'

Bohrmann pressed the panel at the front of his exosuit.

'I didn't want to alarm you,' said Frost. 'I thought if I told you what they're for, you might get nervous and keep looking around for-'

Two torpedo-shaped bodies shot out from behind the scaffold. Bohrmann saw a pair of oddly formed heads. The creatures were coming straight for them, travelling at tremendous speed, teeth grinning in their open jaws. Fear clutched his heart. Bohrmann pushed off from the terrace, moving backwards and shielding his helmet with his hands. None of the movements made sense, but his civilised, scientific mind had yielded to primeval instinct. He cried out.

'They can't hurt you,' Frost said firmly.

The creatures were almost upon him when they banked. Bohrmann gasped for air and tried to fight back his panic. Frost swam to his side. 'We tested the PODs in advance, you know,' he said, 'and they definitely work.'

'What the hell is a POD?'

'A Protective Ocean Device. The best shark deterrent there is. It emits an electromagnetic field that acts as a barrier and keeps the sharks at a distance of five metres.'

Bohrmann tried to recover from the shock. The creatures had swum in a wide arc round the back of the scaffold. 'They were closer than five metres,' he said.

'They'll have learned their lesson now. Sharks have highly sensitive electro-receptive organs. The electromagnetic field over-stimulates their sensors and interferes with their nervous system. It causes them unbearable muscle spasms. During the trial run, we used bait to attract white and tiger sharks, then activated the POD. They couldn't get through the field.'

'Dr Bohrmann? Stanley?' That was van Maarten's voice. 'Are you OK?'

'Everything's fine,' said Frost.

'Well, POD or no POD, it's time for you to leave,' said van Maarten.

Bohrmann's eyes scanned the scaffold. He'd known much of what Frost had told him. Distributed around the front of a shark's head were ampullae of Lorenzini, small canals that detected even the weakest electrical pulses, such as those produced by other living creatures. What he hadn't realised was that a POD could sabotage the sensors. 'Those were hammerheads,' he said.

'Great hammerheads. About four metres long, I'd say.'

'Shit.'

'PODs work especially well on them.' Frost chuckled, 'with their rectangular heads, they've got more ampullae than any other species.'

'What now?'

He saw a movement. Out of the darkness behind the scaffold the two sharks came back into view. Bohrmann stayed still. He watched the sharks attack. Without swinging their heads as sharks usually do when they are tracking a scent, they shot purposefully through the water and stopped suddenly as if they'd hit a wall. They turned in confusion and swam away, then came back and circled the divers, but at a respectful distance.

It worked.

Their body shape was like that of any other shark. It was the head that had given the species its name. It extended on each side in flat wings, with the eyes and nostrils at the far ends. 'The front edge of the hammer was as smooth and straight as a blade.

Slowly Bohrmann composed himself. The creatures wouldn't even be able to harm them through their suits. But he was keen to get out of there.

'How long will it take us to get back?' he asked.

'Same as it took to get down. We'll swim past the scaffold, activate the hounds, and hold tight for the ride.'

'OK.'

'Don't activate anything until we get there. I don't want to see you on a collision course with the floodlights again.'

'How long will the deterrent last?'

'The PODs have at least four hours' worth of battery.' Frost rose through the water, kicking evenly with his fins, holding the console of the trackhound in his right-hand manipulator. Bohrmann followed.

'Well, so long, guys,' said Frost. 'It's too bad we've got to leave.'

The sharks gave chase, but their mouths started to twitch and their bodies contorted. Frost laughed and carried on paddling towards the lighting unit. Against the backdrop of the vast, glowing scaffold, his silhouette looked small and blue-tinged, its contours illuminated.

Bohrmann thought of the blue cloud that had appeared in the distance.

In the shock of the moment he'd forgotten that it had appeared immediately before the sharks had arrived. The same phenomenon had been responsible for the change in the whales and probably for a string of other anomalies and catastrophes. It meant they weren't dealing with ordinary sharks.

Why had the sharks been there in the first place? They had excellent hearing. Maybe the explosion had attracted them. But why were they on the attack? Neither he nor Frost was giving off a scent. They bore no resemblance to prey. In any case, sharks didn't usually attack humans in the depths.

They were approaching the uppermost edge of the scaffold.

'Stan? There's something wrong with them.'

'They won't hurt you.'

I'm telling you, they're not normal.'

One of the sharks turned its broad flat head and swam off to the side.

'You may have a point,' mused Frost. 'It's the depth that bothers me.

Great hammerheads have never been known to go deeper than eighty metres. It makes you wonder what they're-'

The shark turned. For a moment it stopped, head raised slightly and back arched, the classic attack position. Sweeping its tail powerfully it raced towards Frost. The volcanologist was so surprised that he didn't try to fend it off. The shark reared up briefly and violently, then swam into the electromagnetic field and rammed Frost with its flank. Frost twirled like a spinning top, arms and legs splayed.