'And the men?'

'Two missing. The others were rescued. Tell me, Dr Anawak, is there any explanation for their behaviour?'

Good question, thought Anawak. Dolphins and belugas recognised themselves in the mirror. So, could they think? Could they plan? Could they plan in a way that we could understand? What motivated them? Did whales have a future and a past? What possible reason could they have for ramming or sinking a tug?

Unless the tug had threatened them or their young.

'It just doesn't fit with whales,' he said.

'That's what I thought,' Roberts said helplessly. 'But the crews see it differently. In any case, the bigger tug was also rammed. In the end they managed to attach the cable. This time it didn't come under attack.'

Anawak stared at his feet, searching for an answer. 'Coincidence,' he said. 'A horrible coincidence.'

'Do you really think so?'

'We'd have more chance of working it out if we knew what had happened to the rudder.'

'That's why we've called in the divers,' Roberts told him. 'In a few moments they'll be ready to go down.'

'Did they bring a spare set of equipment?'

'I expect so.'

Anawak nodded. 'I'm going too.'

THE WATER WAS REVOLTING, but it always was in docks. The thick dark liquid contained at least as much dirt as it did water. The bottom was covered with a metre-thick coating of mud, over which swirled a permanent cloud of organic matter and silt. As the waves closed over Anawak's head he asked himself how he was supposed to see anything. He could just about make out the hazy outlines of the two divers in front of him and beyond them a dark, misty patch – the Barrier Queen's hull.

The divers gave him the OK sign. Anawak made a circle with his forefinger and thumb in return. He released the latent air in his dive vest and dropped slowly down the side of the boat. They had only gone a few metres when they switched on their head-torches. Exhaled air bubbled and thundered in Anawak's ears. Little by little the rudder emerged in the half-light. Notched and stained, its plate was bent at an angle. Anawak felt for his depth gauge. Eight metres. Ahead of him, the divers disappeared behind the rudder, leaving two stray beams of light flitting through the darkness.

Anawak approached the rudder from the other side.

At first he could see only raised edges and irregular hollows. Then it hit him. The rudder was encrusted with black-and-white mussels. He swam closer for a better look. At the bottom of the rudder, where the plate swept the shaft, the mussels had been ground to pieces. A thick gritty paste filled the cracks and grooves. No wonder it wouldn't respond. It was clogged.

He swam further down the hull. The mussels continued. He reached out gingerly to touch the shells. They were glued to each other in layers, small molluscs no more than three centimetres long. Very carefully, to avoid cutting himself on the edges, he pulled at the mussels until some came loose. They were half open. The fibres that had anchored them in position now poked out of the shells, like tendrils. Anawak stowed them in one of his collection bags, and racked his brains.

His knowledge of molluscs was sketchy. A number of species had a similar-looking byssus, composed of adhesive fibres secreted by the foot. The best known and most feared were the zebra mussels that had been brought over from Asia. In recent years they had colonised the ecosystems of Europe and America, destroying native fauna. If the mussels that had infested the Barrier Queen were zebras, it would explain why there were so many of them. They could establish themselves in no time, spreading at an alarming rate.

Anawak prodded the creatures with his finger. So, the rudder had been invaded by zebra mussels. It seemed the only explanation. But how? They usually preferred a fresh-water habitat. They could survive and reproduce in salt water, but that didn't explain how they could overrun a moving vessel miles from the seabed in the middle of nowhere. Had they latched on to it before it set sail?

The freighter had been en route from Japan. Did Japan have a problem with zebra mussels?

Further down the stern, two curved blades loomed up like ghostly apparitions from the murk below. Anawak swam towards them, kicking his fins until he could grip the edge of one. The propeller measured four and a half metres in diameter. Eight tonnes of solid steel. For a moment he imagined what it would be like when it was turning at full speed. It seemed impossible that anything could so much as scratch it without being shredded.

Yet the propeller was covered with mussels.

An unpleasant possibility occurred to Anawak. Hanging off the edge of the blade, he swung himself hand over hand towards the middle of the propeller. His fingers touched something slippery. Gobbets of a light-coloured substance slid off and floated towards him. He snatched at them, caught one and peered at it.

It was jelly-like, rubbery, and looked like animal tissue.

He stashed it in a collection jar, and felt his way forward. One of the divers appeared on the far side, lamp shining on his mask, making him look oddly alien. He signalled for Anawak to follow. Anawak glided between the rudder shaft and the propeller. He stopped and let himself sink through the water until his fins touched the propeller shaft. A film of slime coated it. The divers were trying to tear it off and Anawak joined in, but they were wasting their energy. It had wrapped itself round the propeller and, without proper tools, they couldn't pull it off.

Anawak thought back to what Roberts had told him. The whales had tried to get rid of the tugs. It was absurd. Why would they sabotage a tow line? So that the freighter would sink? In rougher conditions she might easily have gone under – after all, she was effectively disabled. The sea wouldn't have stayed calm forever. Had the whales been trying to stop her reaching safe water before the weather changed?

He glanced at his gauge. Still plenty of oxygen. He signalled to the divers that he wanted to inspect the hull, and the three left the propeller, fanning out along the side of the vessel, with Anawak at the bottom, where the hull curved round to the keel. The beam of his head-torch explored the steel casing. The paint looked relatively new, with few scratches and little discoloration. He dropped down towards the seabed, further into the gloom. His eyes darted back to the surface: two hazy spots of light marked the position of the divers inspecting the hull. There was nothing to worry about. He knew where he was. All the same he had a heavy feeling in his chest. He kicked his fins a few times and drifted along the hull. No sign of any damage.

All of a sudden his head-torch dimmed. Anawak's hand flew up to check it – but the problem wasn't the lamp: it was where the lamp was shining. Further up, the beam had reflected evenly. Now it was swallowed by a bed of jagged mussels, whose dark outline obscured the painted stern.

How had they got there?

For a moment Anawak considered swimming back to the others, then decided against it and continued down the hull. As he neared the keel the laver of mussels thickened. If the rest of the underside was covered to that extent, a significant weight had accumulated. But someone must have noticed the state the ship was in. On the high seas a load like that would slow a freighter noticeably.

He reached the point where he was obliged to swim on his back. A few metres beneath him lay the muddy wasteland of the harbour floor. He could barely see anything, the water was so murky – just the huge mound of mussels above him. Kicking rapidly with his fins he swam towards the bow. Suddenly the bed of mussels stopped as abruptly as it had begun. For the first time he realised the true size of the outcrop. The mussels had formed a layer two metres thick along the bottom of the Barrier Queen.