Amber heard Li shouting but it seemed to come from somewhere far away. She stared into the gaping mouth of the shark, then closed her eyes and waited to die.

Li grabbed her by the shoulders and yanked her backwards at the same time as Alex hauled Paulo out of the way. The shark bit down on the tin, crumpling it like a chocolate wrapper, then threshed violently, shaking the tin from side to side and smashing repeatedly against the little boat.

The first jolt knocked Alex, Hex and Li off their feet and they fell on top of Paulo and Amber in the bottom of the boat. For a few, nightmare seconds, all they could do was lie in a tangle of arms and legs, clinging on as the boat bucked and tilted with every wood-splintering impact. Seawater poured in on them as the gunwales dipped below the surface of the sea. They scrambled to brace their feet against the side of the boat and threw their weight backwards, desperate to stop the boat from capsizing and tipping them into the water. For what seemed like an age, the boat hung in the balance, then it slammed back down into the water and instantly began tilting the other way, towards the shark.

The gunwales dipped down and more water poured in on them as the shark threshed the sea into a churning foam. It seemed certain that the boat would go over, throwing them to their deaths.

Then suddenly, the threshing stopped. The boat righted itself and wallowed low in the water as the great dorsal fin of the shark rose out of the sea and glided past. It was close enough for them to pick out the scars and nicks and parasitic worms that were scattered across the fin. The tough hide of the shark scraped against the boat like sandpaper as it swam past. They lay in the shuddering boat, holding their breath and listening as the whole length of the shark scraped against the wood. Just as it seemed the shark would go on for ever, the scraping stopped.

Slowly, they sat up, then kneeled in the water-filled boat and peered over the side. The shark – and the storage tin – had completely disappeared. Shakily, they clambered to their feet and collapsed onto the wooden seats. Everyone was dripping wet and shivering with shock. Hex was bleeding from a cut above his eyebrow and the red blood stood out sharply against his white face.

'Dios Mio,' whispered Paulo.

'Carcharodon carcharias,' said Li, faintly.

'What did you call it?' asked Alex.

'That's its Latin name,' said Li. 'It was a great white shark.'

'A great white?' moaned Amber.

'It was a big one,' said Li. 'Must've been over four metres long.'

'Where on earth did it come from?' said Hex, scanning the empty sea. 'And where did it go?'

'I'm not sure it has gone,' said Li, grimly. 'They come to hunt in warm-water currents like this one, because there's always plenty of food here. My guess is, it's been following us, trying to decide what we are. When Paulo emptied the chicken bones over the side, it caught the scent of meat and decided we must be food.'

'I think I'm going to be sick,' gulped Amber, scrabbling for the side of the boat.

Li's eyes sharpened. She grabbed Amber by the shoulders and turned her away from the side. 'No! You mustn't be sick. Not into the sea. Sharks have a fantastic sense of smell. They can pick up the tiniest trace of something interesting, even if it's miles away.'

Li stared into Amber's eyes, willing her not to be sick. Amber swallowed convulsively several times, then her shoulders relaxed and a look of relief crossed her face. 'I'm OK now,' she said.

'Well done,' said Li, giving Amber a smile. 'The last thing we want is that monster coming back.'

'Yeah,' said Alex, leaning over the side to look at the damage to the boat. The bows were dented and splintered and the paint had been scraped away all along the side. 'I don't think we would survive a second attack.'

'Oh, that wasn't a proper attack,' said Li. 'It was going for the storage tin, not the boat.'

'But what about the way it was bashing against the side?' asked Paulo.

'That's instinctive,' said Li. 'Once a shark clamps its teeth down on something, it shakes its head from side to side. The teeth act like a saw, cutting through the flesh so the shark can tear off a big chunk-'

'Whoa! Too much detail!' protested Paulo.

Li blinked. 'Sorry,' she said. Sifting through her vast knowledge of wildlife and pulling out shark titbits had kept the shock at bay. Now, with nothing to fill her mind, the fear came flooding in and she shrank down in her seat, hunching her shoulders.

'What if it comes back?' demanded Amber.

There was a silence as they all thought about that. Alex stood up to get a better view of the surrounding sea while Hex sat in his place and leaned over the side of the boat to inspect the damage.

'What are the odds?' asked Alex, looking questioningly at Li.

'It probably won't,' said Li, finally. 'That tin must've been disappointing food for a shark – and there's nothing else here to attract it…'

She tailed off and stared in horror at Hex, who was still leaning over the side of the boat, looking at the damage. Time seemed to slow down as she watched a bright bead of blood fall from the cut above his eyebrow and drop into the sea. A second bead dropped, then a third as slowly, slowly, she drew a breath and opened her mouth to shout.

'Hex! Get back!' she screamed.

Hex jumped and pulled back into the boat, looking about him wildly.

'Blood!' shouted Li. 'Blood from your head! In the water!'

Hex lifted a hand to his head, then stared at his fingers wonderingly as they came away red. 'I didn't know,' he said.

'But it was only a few drops,' said Paulo. 'A few little drops in a great big sea. The shark, it will not notice-'

'Oh, yes it will,' interrupted Li. 'A shark is a – a hunting machine and it hunts by smell. Most of its brain is devoted to picking up and tracking the scent of prey. To a shark, a few drops of blood are like a very loud dinner-bell.'

'And we're the dish of the day,' said Hex.

Silence fell as they huddled together in the water-logged boat. Nobody moved, except for the constant turning of their heads as they scanned the water. The sea remained empty and quiet.

'I think we got away with it,' said Alex, eventually. He took off his cap and was about to start bailing out the boat when there was a splash behind them. Alex felt his heart jump as he turned to face the stern. The crushed storage tin was bobbing on the surface. Amber began to moan and Paulo put an arm around her shoulders, trying to comfort her.

Any advice, Li?' said Alex, getting to his feet and searching the water around the boat.

Li reached down and pulled the oars out from under the seats. She threw one to Alex and gripped the other firmly with both hands. All her earlier fear had disappeared now that she had something to do.

'If it comes for us, try to whack it hard across the snout,' she said. 'Their snouts are like one big radar device for picking up vibrations, pressure changes and magnetic fields. A good whack might just deter it.'

'OK,' said Alex.

'Oh, and if we can, we should wait until it turns onto its side,' said Li. 'Then aim for the underside of the snout. That's the most sensitive part.'

'There it is,' said Hex, tightly, as the triangular dorsal fin rose out of the sea, cutting a path through the water. Under the surface, they could just make out the huge, grey bulk of the shark in the clear water as it circled closer and closer to the boat. Li groaned as she watched the circling pattern.

'What?' asked Alex.

'That circling is classic hunting behaviour. It means the shark is going to attack the boat,' said Li. 'This time, it's for real.'

'Then let's be ready for it,' said Alex, simply.