'Now,' said Bohrmann.

The seabed rushed towards the screen. For a moment it looked as though the worms had risen to welcome the camera, then it went black. The iron jaws buried themselves in the methane and clamped shut.

'What the hell… ?' gasped the man at the controls.

The numbers on the panel were turning rapidly. They stopped briefly, then sped on.

'The grab's broken through. It's sinking.'

Hvistendahl pushed his way to the front. 'What's going on?'

'This can't be happening! There's no resistance!'

'Pull it up!' screamed Bohrmann. 'Quickly!'

The man jerked back the joystick. The counter stopped, and the numbers started to decrease. The grab rose upwards, jaws clenched. Its external cameras showed the vast hole that had opened. Swollen bubbles surged from inside it. Then a stream of gas gushed out, hitting the grab and engulfing it. Everything vanished in a seething whirlpool.

Greenland Sea

A few hundred kilometres north of the Sonne, Karen Weaver had just stopped counting. Fifty laps of the deck. She kept running up and down, careful not to get in the scientists' way. For once she was pleased that Lukas Bauer didn't have time to talk to her. She needed exercise, but the possibilities on board a research vessel were limited. She'd tried the gym, but the three exercise machines had driven her crazy so she was running instead. Up and down the deck – past Bauer's assistants, who were working on float number five, and past the crew, who were hard at work or standing in groups, watching her, suggestive comments on the tip of their tongues.

Puffs of white breath rose from her parted lips.

Up and down the deck.

She'd have to work on her stamina. It was her weak point. She made up for it in strength, though. Her body was like a sculpture: impressive muscles and glowing skin, with an intricately tattooed falcon between her shoulders. Yet Karen Weaver had none of the bulk of a female body-builder- in fact, she'd have made a perfect model, if only she had been a little taller and her shoulders less broad. A small, sinewy panther, she lived on adrenaline. Her favoured habitat was the edge of the abyss.

In this case, the drop was 3.5 kilometres. The Juno was sailing over the Greenland abyssal plain, an expanse of seabed beneath the Fram Strait, from which the cold Arctic water flowed south. The basin between Iceland, Greenland, the north Norwegian coast and Svalhard was one of the planet's two main water pumps. Bauer was interested in what was going on there – and so was Karen Weaver, on behalf of her readers.

Bauer beckoned for her to join him. With his bald head, huge glasses and pointed white beard, he resembled the cliché of an absent-minded professor more than any other scientist she'd met. He was sixty and already slightly hunched, but indefatigably energetic. Weaver respected people like Lukas Bauer. There was something almost superhuman about them. She admired them for their will.

'Take a look at this, Karen,' he called, in a clear voice. 'Incredible, isn't it? The water here is surging downwards at a rate of seventeen million cubic metres per second. Seventeen million!'' He beamed at her. 'That's twenty times the volume of all the rivers on Earth.'

'Dr Bauer.' Weaver placed a hand on his arm. That's the fourth time you've told me that.'

Bauer blinked. 'Really?'

'And you still haven't got round to explaining how the floats work. You're going to have to talk me through this, if you want me to do your PR.'

'Yes… Well, the floats – that is to say, the autonomous drifting profilers – they… Oh, but you know all that already, don't you? It's why you're here.'

'I'm here to make computer simulations of the currents, so people can see where the floats are going, remember?'

'Of course. Dear me, you can't possibly know… You don't even… Well, I'm a bit short of time, unfortunately. There's so much to do. Why don't you watch for a while and then-'

'Dr Bauer! Not again. You promised to tell me how they work.'

'Certainly. You see, in my articles, I-'

'Dr Bauer, I've read your articles but I trained as a scientist and even I barely understood them. Popular science is supposed to be entertaining. You've got to write in a language that everyone can follow.'

Bauer looked hurt. 'My articles are easy to follow.'

'For you, maybe – and the two dozen others working in your field.'

'Now, that's not true. If you read the text carefully-'

'No, Dr Bauer, I want you to explain it.'

Bauer frowned, then smiled indulgently. 'If any of my students were to talk. . . But they wouldn't dare. They're not allowed to interrupt me – I leave that to myself. He raised his skinny shoulders in a shrug. 'But that's life, I suppose. I can't refuse you anything. I like you, Karen. You're a… Well. . . You remind me of. . . Oh, never mind. Let's take a look at this float.'

'And when we've done that, we'll talk about your findings. I'm getting enquiries.'

'Where from?'

'Magazines, TV programmes, institutes.'

'How interesting.'

'It's not interesting, it's normal – publicity's logical outcome. Do you even see the point of PR?'

Bauer grinned mischievously. 'Perhaps you'd like to explain it?'

'With pleasure – it'd only be the tenth time. But first, you're going to talk to me!

'But that won't do,' said Bauer, in agitation. 'We've got floats to lower, and then I mustn't forget to-'

'Keep your word and talk to me,' Weaver said sternly.

'But, Karen, my dear, you're not the only one getting enquiries. I'm writing to scientists all over the world. They ask the most outlandish things. One just emailed to ask about a worm. Imagine that – a worm! He even wanted to know if the methane concentration was higher than usual, which, of course, it is… But how was he to know? I'll have to-'

'I can deal with all that. I'll be your co-conspirator.'

'As soon as I've-'

'That's if you really like me.'

Bauer's eyes widened. 'I see. So that's how it is, is it?' His drooping shoulders shook with muffled laughter. 'That's exactly why I never married. It's constant blackmail. All right, then, I'll try harder, I promise. Now, let's get going. Come along!'

Weaver followed him. The drifting profiler was dangling from the boom above the grey surface of the water. It was several metres long and protected by a supporting frame. More than half of it was made up of a thin, shiny tube, with two spherical glass containers at the top.

Bauer rubbed his hands together. His down jacket was several sizes too big for him and made him look like an exotic Arctic bird. 'We drop the float into the water,' he said, 'and it bobs along with the current. Think of it as an enormous particle of water. There's a vertical drop beneath us – the water is sinking, as I said… Well, you can't see it sinking, of course, but it's sinking nonetheless. Now, how can I explain this?'

'Try avoiding jargon.'

Right. It's actually very simple. The point is, water doesn't always weigh the same. Warm fresh water is light. Salt water is usually heavier than fresh water. The saltier, the heavier, in fact – there's the added weight of the salt to consider. On the other hand, cold water is heavier than warm water because its density is higher. So water gets heavier as it cools.'

'Which means the heaviest water is always cold and salty,' Weaver put in.

'Very good.' Bauer seemed pleased with her. 'So, water doesn't just flow in currents: it moves up and down in layers. The coldest currents are on the seabed, warm currents are on the surface, and deep-water currents are somewhere in between. Of course, warm currents can travel thousands of kilometres on the surface before they reach colder regions where they start to cool down. And as the water cools-'