By now the scientists and crew were in a state of agitation. Not everyone understood the significance of the smell and the fizzing, but the rough seas compounded the mood of anxiety. Like a vengeful god the storm had swept down from the heavens, hurling towering waves across the sea. It would soon be impossible to stay upright on deck.

Alban had to weigh up all the factors and decide what to do. It wasn't a question of looking at the safety of the research vessel from the viewpoint of the shipping line or as an asset to science. The safety of the Thorvaldson meant the safety of human lives – including those of the two men on the submersible, whose fate Alban's instincts had accepted, even though his mind had not. Both staying and leaving were equally wrong- and right.

He squinted up at the dark sky and wiped the rain off his face. At that moment a brief calm descended on the choppy sea. But the storm wasn't abating, just drawing breath before it continued with increased force.

Alban decided to stay.

IN THE DEPTHS, disaster had struck.

The hydrates, transformed by the worms and bacteria from stable icefields and veins to porous, brittle tatters, had suddenly disintegrated. Across 150 kilometres of the slope, the ice crystals of water and methane broke open explosively, releasing the gas. While Alban was persuading himself to stay, the gas was rushing upwards, breaking through walls, cracking rocks, and causing the shelf to rise up and slide forward. Cubic kilometres of stone caved in. As more layers collapsed further down, the whole seabed along the shelf margin was thrown into motion and started to slide. A violent chain reaction pulled one landslide after the another, as debris rained down on the remaining stable layers, reducing them to mud.

The shelf between Scotland and Norway, with its oil wells, pipelines and platforms, showed signs of cracking.

Someone shouted through the storm at Alban. He spun round and saw the chief scientist waving at him frantically. He could barely hear the man's words. 'The slope,' was all he could make out. 'The slope.'

After the lull the sea had whipped up into a frenzy. Black waves battered the Thorvaldson. Alban looked despairingly at the boom from which the Deep Rover had been lowered into the water. The water was foaming. The stench of methane was overpowering. He started to run amidships where the scientist grabbed his arm.

'This way, Alban! Oh, God – there's something you should see.'

The ship shook. Alban heard a low rumble. It was coming from beneath the water. They staggered up the narrow swaying stairway to the bridge.

'Look!'

Alban stared at the control panel. The sonar was scanning the depths. He couldn't believe his eyes. The seabed was gone. It was as though he was looking at a maelstrom. 'The slope's collapsing,' he whispered.

At that moment he knew there was nothing he could do for Eddie or Stone. His foreboding had become a dreadful certainty. 'We've got to get out of here,' he said. 'At once.'

'But which direction?' the helmsman asked.

Alban tried to think. He knew what was happening down there and what lay in store. Heading for port was out of the question. The Thorvaldsons' only chance was to sail out to sea as fast as she could. 'Put out a radio message,' he said. 'Norway, Scotland, Iceland and all the other North Sea states. They need to evacuate their coasts. Do it now. Get through to everyone you can.'

'But what about Stone and-' the scientist began.

'They're dead.'

He didn't dare think about how powerful the slide might have been. The images on the monitor had sent shivers down his spine. And they weren't out of danger yet. A few kilometres closer to the shore and the ship would capsize. Further out to sea there was a chance they'd escape, despite the fury of the storm.

Alban tried to remember how the slope was shaped. Towards the north-west the seabed descended downwards in a series of large terraces. If they were lucky, the avalanche would come to rest before it reached the bottom. But there was no stopping a Storegga Slide. The whole slope, hundreds of kilometres of it, would slip into the depths, descending 3500 metres. The slide would penetrate as far as the abyssal plains east of Iceland, sending apocalyptic tremors through the North Sea and the Norwegian Sea.

Alban looked up from the console. 'Head for Iceland,' he ordered.

WHEN THE FIRST branch of the avalanche reached the Faroe-Shetland Channel, the submarine terraces between Scotland and the Norwegian Trench had already disappeared, transformed into a slurry of debris that gathered pace as it crashed into the depths, pulling with it everything in path. Nothing of shape or structure survived in its wake. One part of the avalanche split off to the west of the Faroe Islands and came to a halt in the underwater banks surrounding the Icelandic Basin. Another part headed along the mountain range between Iceland and the Faroes.

But the bulk thundered down the Faroe-Shetland Channel as though it were a chute. The same basin that had absorbed the Storegga Slide thousands of years earlier was filled by an even bigger avalanche, pushing forwards relentlessly.

Then the edge of the shelf broke away.

Over a stretch of fifty kilometres the shelf snapped off. And that was just the start.

SVEGGESUNDET, Norway

Once the helicopter had taken off, Tina Lund had loaded her luggage into Johanson's jeep and driven away fast through the rain. God knows what Johanson would have said, but Lund believed in pushing a car to its maximum.

A weight lifted from her mind with every passing kilometre. It had all clicked into place. Once she'd cleared up that business with Stone, she'd called Kare and volunteered to spend a few days with him on the coast. Kare had seemed pleased, if a little bewildered. Something in his voice made her suspect that Johanson had been right and that she'd settled on the right course just in time.

As the jeep rolled down Sveggesundet's high street towards the seafront, her pulse quickened. She left the vehicle in a car park just along from the Fiskehuset. A track and a path led down towards the sea. It didn't look like a typical beach: the boulders and slabs of stone were covered with moss and ferns. Although the area around Sveggesundet was flat, it was wild and romantic, and the view from the Fiskehuset, with its dining terrace on the front, was impressive – even on a misty, rainy morning like today.

Lund strolled to the restaurant and went in. Kare was out, and they hadn't started serving. A kitchen assistant walked past with a crate of vegetables, and told her that the boss had business in town. He didn't know when he'd be back.

It's your own fault, Lund told herself.

They'd agreed to meet there, but- probably because she'd driven like a mad thing – she was an hour early. She'd just have to sit and wait.

She stepped out onto the terrace. Rain pelted her face. Some people would have fled indoors, but Lund barely noticed it. She'd spent her childhood in the country. Sunny days were wonderful, but she enjoyed rain and gales too. Suddenly it occurred to her that the gusts that had rocked the jeep for the final half-hour of the journey had turned into a severe storm. The mist had thinned, but the clouds had sunk and were scudding low across the sky. White spray billowed from the furrowed sea.

Something wasn't right about it.

She'd been here often enough to know the place quite well, but now the beach looked longer than usual. The pebbles and rocks seemed to extend forever, despite the crashing waves. Like an impromptu ebb tide, she thought.

On impulse she pulled out her mobile and called Kare. She might as well tell him she had arrived – rather than risk taking him by surprise. She didn't want anything to go wrong.