NIM, EDMUND AND Tristan were still brushing and polishing the sea turtle fossil so the opal gleamed brighter and its fires leapt further. Fred was scrambling from one person’s shoulder to another, trying to catch glow-worms.

Suddenly Tristan froze. ‘I can hear something.’

They all stopped and listened. They could hear their own breathing; nothing else.

‘I’m going out to see,’ said Tristan.

‘I’m going on working,’ said Nim, and Edmund agreed. The turtle was so beautiful that they wanted it to be perfect, and every time they brushed one bit, another bit that needed brushing showed through.

Tristan crouched through the tunnel and crawled out the door hole. The sunshine was blinding after the hours of darkness; he leaned against the side of the hill as he waited for his eyes to work again, and listened.

He’d told Nim and Edmund that he’d heard something because he didn’t know how else to explain the feeling. It was like a warning siren inside him: Something’s wrong! Pay attention. But outside, he could almost hear it with his ears. It was a very faint, gasping noise, like the sound that Ollie made when he’d cried so much he couldn’t cry any more.

The only other time Tristan had ever felt anything like this was when he and Tiff were seven. He’d been at a friend’s house when Tiff had fallen off the monkey bars and broken her arm. Tris hadn’t waited to tell his friend he was leaving: he’d just run all the way to the playground. He’d got there just as his mum’s car disappeared around the corner on the way to the hospital.

He’d never forgotten that feeling.

So he headed down the hill now, still listening. Whatever it was sounded more and more like sobbing, and by the time he got to the Emergency Cave, Tristan was running so fast he skidded right past the entrance.

‘Tris?’

He spun around. His sister and brother were huddled on the floor.

‘How come you weren’t here before?’ Ollie demanded. ‘We were going to give you a big surprise!’

‘You did,’ said Tristan.

Tiffany sniffed, wiped her nose on the back of her arm, and stood up. ‘Something terrible’s happened. Lance and Leonora want whatever it is you’ve found, and they say nothing’s going to stop them.’

TRISTAN LED THE way back up the path, faster than they’d ever thought they could walk at the top of a cliff. He turned where two sticks lay crossed in front of a tree, and disappeared through the hole in the rock.

‘I thought I’d looked everywhere,’ Tiffany said bitterly. ‘But I went right past it!’ She boosted Ollie in through the hole, and slid in after him.

There was a boom of thunder. The first fat raindrops chased her through the hole into suffocating darkness. If she hadn’t been tied to her little brother Tiff would have climbed right back out again. She’d rather be wet than cramped in a low, dank tunnel.

Tristan turned on his torch. ‘The fossil is on the other side of the wall, right about here.’

‘But we’ve got to go down the tunnel to get there?’ Tiffany asked, pulling her own torch out of her pocket.

Before she’d had time to switch it on, Tris and his light turned into the side tunnel. Ollie followed close to his brother, and for a second, as Tiff stepped into the vast, eerily glimmering cavern, she was alone. She forgot the bats, she forgot Lance and Leonora, she forgot to be afraid. She felt as if she’d stepped into an enchanted world, and she didn’t know yet if it was good or evil.

Then the sheet around her waist tugged her along, and she hurried around the bend.

Nim and Edmund stepped back in surprise as Tristan appeared with Ollie. Their lights shone on the huge opal turtle, and when Tristan added his, the blues and greens gleamed as if the sea was caught in the rock, with hints of fire flashing from the heart of Fire Mountain itself.

Nim prickled. She didn’t want Tiffany here. This was the most special thing she’d ever discovered. Every time she rubbed another little bit clear – another vertebra in the turtle’s neck, or a deeper patch of colour on the shell, excitement bubbled up in her all over again. Now Tiffany was going to say something sarcastic and wreck that feeling.

‘Wow,’ Tiffany breathed, exactly like Tristan had. ‘That’s the coolest thing I’ve ever seen.’

Nim waited. ‘Really?’ she asked at last.

‘Totally,’ said Tiffany. She touched the turtle’s shell with a careful finger, as if she were stroking a butterfly – and the coldness of the touch seemed to wake her up.

‘This is terrible!’ she said urgently.

Nim bristled again.

‘It’s so amazing, the Bijous are going to want it for sure. They’re coming to find it.’

‘The crossed sticks!’ Tristan interrupted. He was sure they hadn’t been at the entrance this morning. Lance and Leonora must have marked it. He raced back down the tunnel.

‘They know you’ve found something,’ Tiffany went on. ‘They’re coming back with gear so they can take it.’

‘They wouldn’t do that!’ Nim exclaimed. ‘They know the island’s protected.’

‘I heard them when they came back to the camp,’ Tiffany said desperately. ‘They want to destroy Jack’s research, too. You’ve got to believe me, Nim.’

‘But …’ Nim began.

‘I’ve just worked something else out,’ said Edmund. ‘When we were on our way to the boat yesterday, Lance said, Let’s hope the cake was worth it. Leonora told him to be quiet and changed the subject really fast – I think he’d forgotten I was in the back seat. But they must have been the scientist friends who gave Dr Ashburn and Professor Hunterstone a celebration dinner. What if they poisoned the cake?’

That’s why Alex said, ‘Ah!’ like that when I said the Bijous were the same kinds of scientists, Nim thought. She meant it reminded her of a plot for a mystery story!

‘That makes sense,’ said Tiffany. ‘They said all our parents were sweet and stupid.’

‘Jack too?’ asked Nim.

‘They said he was so simple it was as if he wanted his research destroyed.’

Nim knew that Tiffany was telling the truth.

Tristan rushed back in. ‘Lance and Leonora have stolen the boat! It’s down in that little cove below us – and they’re climbing the cliff. I was too late to move the sticks, they’d have seen me for sure.’

‘We’ll guard the tunnel,’ Nim said fiercely. ‘We’re not letting them in!’

‘We’ll have to come out some time – and they’ve just hauled a big pack up here in pouring rain,’ said Tristan.

‘They’re not going to go away just because we ask them to.’ ‘The way they said that nothing was going to stop them …’ Tiffany shivered.

Ollie stared from one face to the other, and started to whimper. Fred scuttled back to Nim’s shoulder.

Nim felt as if she’d swallowed a hurricane. Her thoughts whirled: she had to stop the Bijous, but she couldn’t put everyone in danger; she had to stop them destroying Jack’s research, but she had to guard the cave and the bats’ nursery.

‘We can’t fight them while we’ve got Ollie,’ said Tiffany, as if she’d read Nim’s mind.

‘But it’s too late to run away,’ said Tristan.

‘The cavern has so many tunnels, there must be another way out,’ said Nim. ‘We can take Ollie somewhere safe and then I’ll come back to …’

‘I’ll come back with you,’ Edmund interrupted.

‘So will I,’ said Tris.

‘I would if I could,’ said Tiff.

‘We need you to be safe outside with Ollie,’ said Nim, ‘because, just in case … if anything happened … you could go for help when Jack and your parents get back.’

That was the scariest thing of all. Saying out loud that something could go wrong meant it really could.

Nim led the others back through the fossil tunnel and back into the main cave. Silently, they tiptoed beneath the dangling, sleeping bats, around the maze of stalagmites, deeper and deeper into the heart of the mountain. The glow-worms disappeared: the dark was darker, the air was danker, and the musty bat smell wasn’t getting any weaker.

Maybe there isn’t another way out, a voice whispered in Nim’s head. Maybe you’ll be trapped here forever.

We’re not trapped, she told it. If we can’t find another tunnel we’ll just go back. Lance and Leonora are scientists. They’re not going to hurt us.

The dim glow from her headlamp shone into a different darkness. She’d found another tunnel.

The floor was moist and slippery. It sloped downhill more steeply than the cave, and was deeper in the middle than the sides, as if it had been neatly hollowed out. By water, Nim thought. This is the watercourse that comes out above Waterfall Cliffs!

She was about to whisper the good news when the nasty little voice in her head said, Or it’s an underground creek that runs into an underground pool – and you’ll never get out!

Nim decided she wouldn’t say anything yet.

‘At least we can’t get lost now,’ Edmund whispered in her ear.

It was true. The tunnel was narrow and the only other openings were small nooks or alcoves, as if someone had thought about making a window or a door and then remembered that there was nowhere for it to go. All Nim had to do was keep on going, and all the others had to do was follow.

That didn’t stop her stomach squirming with fear. Because even if they weren’t exactly lost, they didn’t know where they were, and they didn’t know where Lance and Leonora were or what they were doing. And even though they were going as quickly as they could, it wasn’t very fast because the tunnel was dark and the ceiling was low. Nim had to walk with her head bowed, and Edmund had to crouch. Ollie was the only one who could walk normally. He was still tied to Tiffany by the sleeping sheet, but he kept rushing ahead, sliding and bumping into Edmund.

Then they were all slipping, skidding down the tunnel. The floor was as slimy as a hungry slug, and so were the walls. They slid down as if they were going down the rainforest waterslide. It would have been fun if it hadn’t been so dark and they hadn’t been so scared of Leonora and Lance hearing them. But it was dark, and they were scared – all except Ollie.

‘I’m going fast!’ he squealed. ‘Fast, fast, FAST!’

‘SHH!’ everyone else hissed.

But Ollie squealed and skidded again, passing Edmund and Nim, and Tiffany skidded behind him. Then his squeal changed to a screech, and there was a slithery crashing noise and a splash of pebbles into water, and then more crashing and a scream from Tiffany.

‘Tiff!’ Tristan shouted.

Tiffany and Ollie had disappeared.