'At your service,' Johanson mumbled. Still dizzy, he dragged himself off to the shower.

Thirty minutes later, he was feeling more alive. Outside, it was sunny and Kirkegata Street was all but deserted. The last piles of snow had melted and as Johanson drove out towards the Gloshaugen campus he was whistling Vivaldi. The university was supposed to be closed at the weekend, but no one paid any attention to the rules: it was the best time to sort your mail and work undisturbed.

Johanson went to the post-room, rummaged in his pigeon-hole and pulled out a thick envelope. It had been sent from Kiel and almost certainly contained the lab results that Lund was so desperate to see. He stowed it away, unopened, went back to his car and resumed his journey to Tyholt.

The Institute for Marine Technology, or Marintek, as it was known, had close links with the NTNU, SINTEF and the Statoil research centre. In addition to its collection of simulation tanks and wave tunnels, it also housed the world's biggest artificial ocean-research basin, offering scientists scale-model testing in simulated wind and waves. The Norwegian shelf was covered with floating production systems that had been tested in the eighty-metre-long by ten-metre-deep pool. Two wave machines created miniature currents and storms that seemed terrifyingly powerful. Johanson was pretty sure that Lund would use it to test the underwater unit that she was planning for the slope.

As he had expected, he found her at the poolside, talking to some scientists. There was something droll about the scene. Divers were weaving through the blue-green water past Toytown platforms, while miniature tankers floated past lab staff in rowing-boats. It resembled a cross between a toy-shop and a boating party, but it had a serious purpose: the offshore industry needed Marintek's blessing before any new structure could be built.

Lund spotted him, broke off her conversation and headed over. It meant walking all the way round the pool, which she did at her usual canter.

'Why not take a boat?' asked Johanson.

'This isn't the village pond, you know,' she said. 'Everything has to be coordinated. If I ploughed willy-nilly through the basin, hundreds of oil workers would die in the tidal wave.'

She gave him a peck on the cheek. 'You're all scratchy.'

'All men with beards are scratchy,' said Johanson. 'It's lucky for you Kare hasn't got one, or you'd have no excuse for picking him instead of me. So, what are you working on? The subsea problem?'

'As best we can – the basin only lets us simulate realistic conditions for depths of up to a thousand metres.'

'You don't need to go deeper.'

'Theoretically, no. But we still like to run through the scenarios on the computer. Sometimes its predictions don't fit the results from the basin, so we keep adjusting the parameters until we get a match.'

'Shell's looking into building a unit two thousand metres down. It was in the papers yesterday. You've got competition.'

'I know. Marintek's doing the research for them too. It'll be an even harder nut to crack. Come on, let's get some breakfast.'

Once they were out in the corridor Johanson said; 'I still don't understand why you can't use a SWOP. Isn't it easier to work on a floating platform and connect it via flexible flowlines?'

She shook her head. 'Too risky. Floating structures still have to be anchored.'

'I know that.'

'And they can always come adrift.'

'But the shelf's full of them!'

'Granted, but only where it's shallower. In deeper water, the waves and currents are different. Besides, it's not just a question of anchoring the units. The longer the riser, the less stable it becomes. The last thing we need is an environmental disaster. And, anyway, who'd want to work on a floating platform on the other side of the shelf? Even the hardiest would spew their guts out. This way.'

They went up some stairs.

'I thought we were going for breakfast,' said Johanson in surprise.

'We are, but there's something I want to show you first.'

Lund pushed open a door and they went into an office on the floor immediately above the ocean basin. The large glass windows looked down on neat rows of sunlit gardens and gabled houses that stretched out in the direction of Trondheim fjord.

She walked over to a desk, pulled up two Formica chairs and flipped open the widescreen laptop. Her fingers drummed impatiently while the program loaded. The screen filled with photos that seemed strangely familiar. They showed a milky-white patch dissolving into darkness at the edges. All of a sudden Johanson realised what he was looking at. 'The footage from Victor,' he said. 'It's that thing we saw on the slope.'

'The thing I was worried about.' Lund nodded.

'Do you know what it is yet?'

'No, but I can tell you what it isn't. It's not a jellyfish, and it's definitely not a shoal. We've tried putting the image through countless different filters, but this is the best we could do.' She enlarged the first photo. 'The thing was caught in Victor's floodlights. We saw a part of it, but not as it would have looked without the artificial lighting.'

'Without the lighting you wouldn't have seen anything. It was far too deep.'

'You reckon?'

'Unless, of course, the thing was bioluminescent, in which case-' He broke off.

Lund appeared pleased with herself. Her fingers danced over the keyboard and the picture changed again. This time they were looking at a section from the top right-hand corner. At the edge of the image, the bright patch dimmed into darkness, and faint marks could be seen. It was a different kind of light, a deep-blue glow streaked with pale lines.

'When light is directed at a luminescent object you can't see its natural glow. Victor's floodlights are so powerful they illuminate everything, except at the very edge of the picture where they're no longer so bright.

But there's definitely something there. That proves to me we're dealing with a luminescent creature – a pretty big one.'

Many deep-sea creatures could luminesce. Their light was the result of symbioses with bacteria. Some organisms on the surface of the ocean could emit light too – algae, for instance, and some small species of squid – but the real sea of lights only started where darkness began, beyond the reach of the sun.

Johanson stared at the screen. There was only a hint of blue, barely visible, and most people would have missed it. Still, the robot was known for the high resolution of its pictures. Perhaps Lund was right. He scratched his beard. 'How big is it, do you think?'

'It's difficult to say because it disappeared so quickly, but in that time it must have swum to the edge of the beam. If you look here, though, it still almost covers the frame, which suggests…'

'That the part we're looking at measures ten to twelve square metres.'

'Exactly. The part! She paused. 'Judging by the light at the edge of the picture, I'd say we saw just a fraction of it.'

A different explanation occurred to Johanson. 'It could be planktonic organisms,' he said. 'Micro-organisms of some kind. Plenty of species glow.'

'How do you explain the markings?'

'You mean the paler streaks? Coincidence. We're only assuming that they're markings. We used to think that the channels on Mars were markings too.'

'I'm certain they're not plankton.'

'We can't see well enough to tell.'

'Oh, but we can. Take a look at this.'

Lund called up the next images. The milky patch retreated further into the darkness. It had been visible for less than a second. The pale area of luminescence was still apparent in the second and third frames, but the streaks seemed to shift. By the fourth frame everything had vanished.

'It turned off its light,' Johanson said, amazed. Certain species of squid could communicate with bioluminescence, and it wasn't unusual for them to flip the switch and disappear into darkness when they felt threatened. But this creature was bigger than any known species of squid.