Li tumbled out of the way and the eel slid harmlessly past, still snapping its jaws. It wasn’t trying to attack her; this was how it breathed. She curled around in the water, lifted her video camera to her masked face and filmed it – the pointed, primitive face like a sea-going snake, bubbles escaping through small holes in its head. The body was nearly three metres long from pointed snout to arrow-like tail and as thick as a telegraph pole.

Up above her on the surface was the Fathom Sprite, the eight-metre motorboat they had hired, tethered and flying a blue-and-white flag to show there were divers in the area. She and the four other members of Alpha Force had come to the Fathoms Dive Centre in Curaçao to train in advanced diving techniques. For now, their first afternoon, they were enjoying the island’s coral reef and reminding themselves of the basic rules of diving.

The coral reef was vast, an underwater cliff that stretched down for ever. It was far too deep to explore with basic scuba equipment, so rule number one was to make sure you knew how far down you’d gone. As Li swam around she kept a close eye on the dive computer on her wrist to check that she never went deeper than thirty metres. Later in the week, with different equipment and more training, she hoped they’d be able to go deeper.

So far, though, there was so much to see that Li didn’t feel shortchanged. Her Anglo-Chinese parents were naturalists and she had inherited their love of the natural world. She hung in front of the wall, the gentle current fanning her black hair out in a rippling flame behind her. ‘Wall’ somehow seemed the wrong word. It looked like a bluish, brownish rockery full of plants and weeds, but the coral was actually all made of living animals. She swept her torch over it and the muted colours became bushes of bright red, yellow and jewel-like purples. There were delicate white structures that looked like the vein structures of giant leaves. In between them all were pale ripples of hard corals like human brains. The textures looked so alien and beautiful she wanted to touch them, but some of them were poisonous. In any case, the whole reef was a conservation area and divers were forbidden to touch anything in case they upset the ecosystem.

Diving this reef was like being a bird. She could hover, or move up and down effortlessly with a flick of her fins. With the endless deep blue below her, it was like dreams she’d had of being able to fly. A talented gymnast, martial artist and climber, Li adored anything that felt like defying gravity.

Strange, though. It was no longer as quiet as it had been. As well as the gentle sound of her own breathing she could now hear a deep sort of rumbling sound, like very muffled thunder. She couldn’t say when it had started, but she was sure it hadn’t always been that loud.

Li checked her luminous watch. They had been underwater for about ten minutes. Diving rule number two was to keep an eye on how long you’d been down because you didn’t want to run out of air. But Li felt like she could spend for ever down there.

All the time she had been keeping track of the others. They all carried torches so it was easy to see where they were. That was diving rule number three – don’t go off on your own. You always had a dive-buddy to look out for you. Today, she had two – Paulo and Alex. Time to film them for a while, she thought.

She picked Paulo out easily by his powerful build and dark wavy hair billowing up in a soft halo. He was above her, suspended in a ring of silvery fish, as though he had charmed them out of the reefs. He saw Li pointing the camera up at him and gave a big, theatrical flourish with his hand. The fish parted in a tunnel. He withdrew it again and they closed around him like a giant rotating lampshade.

Typical Paulo, thought Li. He had grown up on a ranch in Argentina and was supremely confident with animals. Here he was, orchestrating the movements of at least two hundred fish. Any minute now he’s going to give a big show-off grin and then his regulator will fall out of his mouth. Or he’ll breathe through his nose. She made sure she got a close-up of his bouffant hair; that would make amusing viewing later.

Alex had been watching. He swam past Li, his blond hair rippling around his face, and copied Paulo’s flamboyant gesture. Li filmed him, knowing what no-nonsense Northumbrian Alex would be saying about Paulo if his dive equipment allowed him to speak: ‘posing as usual’. As he swam off Li took a few quick frames of Alex’s billowy hair too.

Amber and Hex were easy to spot. Hex’s fins were pale in the blue light, edged with black like a dangerous fish; Amber’s were as black as her skin, making her already long legs impossibly sleek. They were swimming as a pair at ninety degrees to the coral wall, as though they had forgotten which way was up. Several fish seemed to have been fooled too and were swimming along beside them. To anyone who knew them they would seem unlikely divebuddies – Amber, a privileged rich girl from America, and Hex, a computer hacker from a rough part of London – but the two had clicked. Now they swam closer to the coral wall, changing direction in sync like a pair of seahorses, bubbles rising from their masked faces like thought clouds. Both had close-cropped short hair – no potential there, Li thought, for embarrassing underwater hairstyle videos.

Amber poked Hex in the back as they swam along. He whirled in the water looking for his attacker, obviously imagining sharks. Amber hovered beside him, the tubes on her air tank quivering and bubbles shooting fast out of her regulator. So that was what someone looked like when they laughed in scuba gear, Li thought. Hex reached across to poke Amber in return and she scooted gracefully away. He gave chase. And still they swam as though the world had been turned on its side.

That was another of the rules of diving; always know which direction you’re going in. It was easy to become disorientated in the water. On night dives particularly, you might think you were surfacing but in fact be swimming endlessly down. All five members of Alpha Force were well trained in navigation techniques. Sometimes their lives had depended on it.

Alex came into Li’s viewfinder again, now swimming alongside a gigantic grouper fish. It was the size of a small car and made him look as though he had swallowed a shrinking potion. In the light from her torch she could see that the fish was orangey red with mottled pale lines. The perfect portrait of Alex. His father was in the SAS and Alex seemed to have found the one sea creature that wore desert-issue camouflage.

The big fish fluttered its tail and darted away. Alex stopped and looked around, puzzled.

Li realized the booming sound was much louder. Paulo’s cloak of silver fish suddenly deserted him and Hex and Amber stopped too. For a moment the five friends looked around at each other questioningly, treading water. The sound had been growing and growing but was now so loud that they could no longer hear their own breathing.

Where was it coming from? When they looked around, all they could see was wide blue sea and the flowing vertical garden.

Then a huge shape loomed over them in the water like a thundercloud. An enormous ship. It rumbled over them, the throbbing of its engines resounding on every metal item in their kit. Paulo, Alex and Hex were pointing up at it, frantically miming a movie camera to Li. They wanted her to video it. What was it with boys and machines? she thought – but to humour them she lifted the camera. A red light flashed. Out of batteries.

The rear of the boat finally came into view, its mighty propeller spinning in a round opening as tall as a man. The sea behind it boiled into tiny white bubbles. Then the sunlight poured back into the sea again.

They looked at the retreating shadow, then Alex tapped his watch. Time to go. They turned and swam upwards. At the top of the wall was a sandy shelf, the start of the shallow waters near the island. The noise was receding but it was still loud, the boom of the ship’s engines like the throbbing of a great heart.

They began to swim back to their boat, the current from the ship’s wake pulling them along. Then, like a shoal of fish, they all stopped suddenly. Twenty metres away in the water they should have seen a black diagonal thread – the anchor line. Instead the thread was waving loose in the water.

They swam over to it. The anchor line had snapped and was curling in the current like a slender eel.

Alex swam upwards. As he surfaced he found himself bobbing around like a cork. The sea was still choppy from the passing ship. But he wasn’t the only thing being tossed around. Where the boat should have been the water was scattered with debris. The Fathom Sprite had been hit – and shattered like a toy.

The tanker was already a good distance away, heading for the white coral cliffs of the island.

Li and the others came up in a rumble of bubbles, exploding onto the surface as though they were coming to the boil. They looked around at the bobbing white pieces of their boat, too stunned even to take their masks off. Paulo felt something knock into him – the boat’s engine, nudging at his back like a questing fish.

‘Mind out!’ Amber’s shrill voice pulled them back to practicalities. Her regulator was dangling over one shoulder and she was holding onto a yellow object like a folded canvas pillow. She found the ripcord and pulled. There was a hiss of compressed air and it inflated, unfolding to three times its size.

Li saw the life raft about to engulf her and dived out of the way. When she broke the surface again the first thing she saw was the tanker. It was running, at full speed, into the white cliffs.

The noise was terrible – a dull metallic boom, then the sound of grinding metal, on and on like it was in slow motion. Everyone froze: Amber, holding onto the ropes on the raft, pulling Hex in; Alex, also in the raft, turning to help Paulo up; Li, only her head out of the water. In that moment, her vision became a split screen, her mask half submerged, half out in the air. Below was the tranquil world of rippling blue with black stingrays banking and turning like cloaks. Above were the clouds, the tropical island – and a huge, rust-spattered tanker full of thick black oil subsiding into the sea . . .

‘We shouldn’t,’ said Amber, ‘unless someone does something silly.’ Nevertheless, she and the others kept their kit on too. The raft didn’t feel nearly as solid as a boat.

Paulo was looking around the raft. It looked like a large children’s paddling pool. ‘Does this thing have a radio?’

Amber shook her head. ‘No. That went down with the boat.’ She unzipped a compartment in the side of the raft and took out two short paddles. ‘This is how we’re going to get home.’ She handed one to Hex, took the other and they began to paddle towards the shore, digging into the water like kayakers.

‘Should we see if anyone on the tanker needs help?’ said Alex, watching the stricken vessel.

‘They’ve got lifeboats,’ said Hex. ‘See, they’re coming out now.’

As he spoke, a crane swung out from the tanker deck to lower a white boat. They could see small figures peering over the edge as the lifeboat was lowered. No sooner had it hit the water than the ship gave another lurch and the entire stern with its living quarters disappeared beneath the surface, leaving only the communication masts visible. The whirling radar antennae sparked and became still.

Now all they could hear was the gentle splash of oars as Hex and Amber paddled in a slow, steady rhythm. Red pressure marks from the masks framed their eyes, making them look tired. Puddles of water collected on the yellow canvas floor.

‘I’ll take next turn at paddling,’ said Paulo.

‘Me too,’ said Li. Her voice was subdued.

Alex looked towards the small white-painted jetty in the distance, and the long wooden building behind it that was the dive school. ‘What on earth are we going to tell Danny about his boat?’

Nobody answered.

They paddled, steadily but surely. It was barely five minutes since they had surfaced and found their boat gone, but they had taken disaster in their stride.

Alpha Force had learned about survival the hard way when they had been five strangers marooned together on a desert island. By the time rescue arrived they had pulled together into a tight-knit team. The experience had been a turning point in their lives – especially for Amber. She had been getting over the deaths of her parents and had discovered that they had been living secret lives fighting human rights abuses. What the five friends went through on that island had shown them that they also could make a difference to those in trouble. Now, they had dual lives. During term time they were in far-flung corners of the globe, at school or college, keeping fit in their spare time and improving their individual skills. In the holidays they came together to put it all into practice with some extreme sports and training. Quite often these training sessions had a habit of leading them into more serious challenges.

They heard the dinghy before they saw it, the sound of its engine carrying over the water like the drone of a bee. A small blob was racing out from the shore, a silver dinghy carrying two figures in orange lifejackets.

Alex squinted into the distance, his hands shielding his eyes from the bright sun. ‘Hey, the coastguard’s out and about.’

‘They’re coming for us,’ said Li.

The dinghy drew up close, its engine cut to idle and a man leaned over to talk to them. He had thinning blond hair and a deep tan from a lifetime spent on boats. His lifejacket was printed with the word COASTGUARD.

‘Anyone hurt?’

‘No, we’re fine,’ Alex confirmed.

As he spoke, Paulo greeted the athletic, ebony-skinned figure with the coastguard. ‘Hi, Danny,’ he said with his warmest smile. Danny was the owner of the dive school, a younger man than the coastguard. ‘I’m really sorry, but I’m afraid something’s happened to the Fathom Sprite.’

The others waited in tense silence. The accident had hardly been their fault but they hadn’t worked out yet how they would break the news. Paulo, with his usual easy charm, had come straight out with it.

‘I guessed,’ said Danny. ‘When we saw you in this.’ His face, normally creased with permanent laugh lines, was grave.

There was a moment of silence as both boats bobbed up and down together on the waves, like horses on a fairground carousel. In the distance, the white lifeboat from the tanker had reached the shore.

The coastguard put his hand on Danny’s shoulder. ‘You can sue the oil company, Danny,’ he said. ‘That tanker was going too fast on an unauthorized course. I’ve got evidence.’

‘Evidence?’ said Alex.

The coastguard nodded. ‘I had a call from a passenger cruiser which was nearly hit by an out-of-control tanker – that tanker. You’re lucky you weren’t in the boat at the time.’ He looked at the raft. ‘Do you guys need a ride? We can give you a tow.’

Paulo picked up a paddle. ‘Actually,’ he said, ‘I was quite enjoying this.’

Amber smiled up at the two men. ‘I think we’re just fine.’

The coastguard gave them a small salute and nodded to Danny. He opened the throttle and they sped away.

The cosy, wood-panelled room was packed. The twenty guests who were staying at the dive centre were there, plus various members of staff. All eyes were on the TV up in the corner. Normally it was only on when there were sporting events and concerts; but today the local news was covering the story of the stricken tanker. On the screen was an aerial picture of the vessel, taken from a helicopter. Badged in one corner with the channel logo, the image changed as the camera circled from the rust-red prow poking out of the water to the tips of the communication masts – all that remained of the stern. Around it, like an ominous shadow, the clear blue water was turning black. Amber swallowed. The oil was already leaking from the ship.

The programme cut back to the studio, where an anchorwoman looked at the camera with a steely eye. ‘Those were pictures today of the ArBonCo tanker disaster. I have here in the studio Dr Mara Thomas, Curaçao representative of the environmental group ABC Guardians and a GP at the local medical centre. Mara, how bad is this?’

The camera panned back to show a strong-jawed Caribbean woman in her forties. ‘It’s a catastrophe,’ the doctor said. ‘It’s a very delicate ecosystem out there. The whole community has worked for years to keep the area clean. The reef provides a livelihood for us all – food, tourism, it all depends on it. This oil will kill so much marine and bird life. Imagine what a nuclear war would do to the city – it’s like that.’ As she spoke her dark curly hair quivered like wire; although it was held in an antique clip it threatened to escape at any moment.

‘Yo, Mara,’ called a voice – Danny, his elbows resting on the rough wooden bar top that looked as though it had come from an old galleon. Behind him, gathering dust with the bottles of local rum on the top shelf, were several trophies. ‘You tell them what it’s doing to us,’ he continued.

On the television, a new image had appeared. A man in a grey suit was waiting patiently and the anchorwoman introduced him: ‘I have here Piers Hijkoop, legal representative of ArBonCo Oil. Piers, these are strong words. How do you respond to these concerns?’

The man replied calmly. ‘We understand the concerns of the locals and very much regret what has happened. Our experts are already at the scene of the accident. They should be able to contain the spill so that the threat to marine life is minimized.’

Danny spoke again. ‘Looks like it’s too late for that.’ There was a murmur of agreement from everyone in the room. Alpha Force recognized the man sitting near him: it was the coastguard they had met earlier.

On the screen the anchorwoman asked Piers Hijkoop: ‘Any clues as to what caused the crash?’

‘Piers Hijkoop, Dr Mara Thomas, thank you very much.’ The anchorwoman turned away from her guests and addressed the camera. ‘We’ll be bringing you more on the story as and when we get updates. And now on to other news—’

Danny fired the remote at the screen. It blinked off. For a moment the only noise in the room was the soft lilt of reggae music on the radio.

A woman joined Danny behind the bar and helped herself to some juice from the fridge. Alpha Force recognized her too – Danny’s American partner, Lynn, who had been a photographer before they decided to set up Fathoms Dive Centre together. She had helped them settle in when they first arrived.

Amber heard her grumble to Danny: ‘Well, Mara didn’t get much of a say.’

The coastguard was leaning over the bar now as well, joining in the conversation. ‘They won’t keep Mara quiet for long.’

Amber was struck by how they spoke – as though they knew Mara personally. Everyone in this community seemed to know each other; they were like one big family. A family on the brink of potential tragedy.

People were heading for the bar and looking for drinks, so Danny and Lynn had to put on their professional faces. Slowly the room was filled with the murmur of conversations.

Alex stood up to give the people at the bar more room. ‘Guys, I think we’re in the way here. Anyone fancy a breath of fresh air?’

The others nodded. They got up and headed for the row of double doors that led out onto the veranda.

Below was the dive school’s private bay, jetty and beach. The sun was setting, like a bonfire behind the mountains on the west side of the island. Most of the white beach was already in darkness and red-orange light glinted off the waves as they surged and ebbed up and down the beach.

Something caught Li’s eye and she went down the wooden steps to the beach. Noticing her body language, the others followed silently and saw what she had seen. Close to, the surface of the water had an iridescent petrol sheen, like oil on the surface of a puddle in a car park. It swirled purple and blue in the light from the setting sun.

Li kneeled down. The sea came all the way up to her feet, then began its retreat, water sinking down between the shards of coral on the beach as it left, winking into bubbles and then into nothing – but not quite nothing. There was now a dark film over the white coral. And a smell.

Hex sniffed. ‘Rotten eggs.’

Li stood up. Another wave came and went, leaving another layer of oil.

Paulo caught the expression on her face. ‘It’s started,’ he said.

The five members of Alpha Force stood on the veranda and looked out at the mess. It was even worse than they’d expected.

‘What a difference,’ said Hex. ‘It’s obscene.’

‘It spreads so fast,’ said Paulo.

And still the sea brought more. The clear blue sparkling water was tainted with a rainbow sheen, the white sandpiles from escaping creatures darkening with every wave.

‘We had an oil spill once on the beach in Northumberland,’ said Alex. ‘A tanker ran aground in a storm. There were dead birds everywhere. I thought we’d see dead birds here.’

Li answered his question in a low voice. ‘We’ll see the birds soon. Give it time. At the moment they’ll be trying to clean oil off their feathers, shivering like fury. But the oil will destroy the waterproofing in their feathers so they’ll get colder and colder. Then they’ll try to eat more but their digestive systems will be irritated by the oil. They’ll start burning up their own body tissues to keep warm. And they’ll keep trying to clean themselves, and all that oil they’re swallowing will poison them. By the time we see them they’ll be desperate.’

Amber’s eyes were starting to water from the fumes. She rubbed them. ‘This stuff is vicious.’

Even the sounds of the landscape were different. There was the usual steady drone of boats, but also the beat of a helicopter. One came close enough for them to see its tail with the red insignia of ArBonCo Oil. Then it disappeared around the headland towards the tanker.

‘I wonder if they really can do anything?’ said Alex.

‘Why haven’t they called us yet?’ said Amber. ‘Hex, you definitely got through and put us on the list of volunteers?’

‘I spoke to them last night,’ said Hex. ‘In person, not a machine.’ He patted his mobile phone on his belt. ‘As soon as the call comes, we’ll know. They said they were waiting for supplies.’

Li let out a long sigh. ‘Surely there must be something we can do now.’

Hex shook his head. ‘They said no one’s to enter the water until the equipment’s here. Otherwise we run the risk of spreading the oil further. Plus the tanker’s unstable and might explode if any air has got in with the oil.’

Paulo winced. ‘Nasty.’

Amber frowned. ‘Surely it’s seawater in there, if anything?’

‘No,’ said Hex. ‘Not all the tanker’s underwater. The containers that aren’t might have been holed. It’s probably just a precaution.’

‘So we just wait?’ said Amber.

‘Yeah,’ said Hex. ‘We just wait.’

Alex moved towards the veranda doors. ‘I’ve done enough waiting. Does anyone feel up to some studying? There’s lots of diving stuff we could revise.’

Amber thought that whoever had tried to make the dive centre library look like part of an old ship had not realized that most of the walls would be covered in books. Not old leather-bound gold-lettered books either; the diving textbooks were full of modern typefaces and clashing colours. Danny kept the place well stocked with the latest publications.

As the five teenagers sat at the big table, surrounded by books, their sense of time dragging disappeared. Training always put them in a positive frame of mind. It was storing up tools that could be useful for some mission in the future, something that would let them do their job better – or even save their lives or the lives of others. They took training very seriously. For a while they almost forgot about the ruined landscape outside.

Hex kept consulting his palmtop, a state-of-the-art computer that was his pride and joy. He carried it everywhere with him in a belt-mounted pouch. His one complaint about doing so much diving was that he had to leave it behind. But he had managed to put it to good use the previous day, taking notes from an in-depth lecture that Danny had given them on ‘the bends’, or decompression sickness. If they dived deeper than thirty metres, the weight of the water forced nitrogen from the air they were breathing to dissolve in their blood and joints. They had to be very careful how long they stayed down – and how fast they surfaced – or the dissolved nitrogen would fizz up in their bloodstreams like the gases in a can of drink. The bubbles might burst blood vessels, rupture lungs or even damage nerves. The deeper they dived, the higher the risk. There was a lot of theory to learn before they could dive safely at these depths, and a lot of maths to practise.

Paulo had found a stack of videotapes. ‘We ought to look at one of these.’ He read off the titles.

‘Nitrogen Narcosis: The Facts; Diving Physiology—’

‘We’ve done those,’ chorused Amber and Li.

‘How about Psychological Preparation for Diving?’

‘Done that too,’ said Alex.

Hex interrupted as something caught his eye in the book he had in front of him. ‘Did you know,’ he said, ‘that decompression sickness was first observed by Robert Boyle in the seventeenth century? He put a viper in a vessel and increased the pressure –’ He put his hand up like a policeman stopping traffic. ‘Don’t ask me how because it doesn’t say. After decreasing the pressure he noticed that a bubble formed in the eye of the snake, and it was writhing in pain.’

Li thought that Hex had a certain air of satisfaction when he closed the book. ‘That’s disgusting,’ she said. ‘I hope the viper bit him.’

Alex blew his cheeks out. ‘I thought all Boyle ever did was write boring old Boyle’s Law.’

‘Hey, guys,’ said Amber. ‘This is so weird. We could all be at my school in study period.’ She glanced at Hex. ‘Except for you. They wouldn’t let you in my school.’

‘I wouldn’t want to come to your school,’ rejoined Hex. ‘It’s just for rich American girls.’

‘I think I’d like it there,’ said Paulo.

‘Oh, listen to him,’ said Li. ‘Mister heartbreaker. If you set foot in a girls’ school they’d have you for breakfast.’

Paulo gave her a smouldering look. ‘They can have me anytime.’ Li picked up her notes and rapped them down on his tanned hand.

‘What’s your place like, Alex?’ Amber asked.

‘Oh – so-so,’ said Alex. ‘Just your usual kind of college. Near the moors. That’s what I like about it.’

Paulo read out another title. ‘Gas Mixes. No, we’ve done that too. Cliff Diving Championships 2004.’ That one took the wind out of his sails.

‘Cliff Diving Championships 2004?’ Alex repeated.

Paulo looked at the video. It wasn’t commercially produced; the label was handwritten in biro. He handed it over to Alex, who was next to the combined TV/video unit, and Alex slotted it into the machine and switched it on. They leaned forward to watch.