TIFFANY’S WORDS EXPLODED over them. Time froze.

Then Tristan shouted, ‘Of course you can get out! You have to.’ Ollie started to sob, and Edmund pointed to a thin stream of water trickling down the tunnel towards them.

The rain that had started when Tiffany got to the cave was flowing all the way through from the entrance hole. There was already water below Tiffany. If more flowed in …

Don’t go down the shaft! Nim wished at the stream, but of course it did.

She looked at Edmund and could tell he was thinking the same thing.

‘I know Lance and Leonora want to steal the fossil,’ said Nim, ‘but they’d have to help if we ask – they’ve got ropes and stuff, and they know about rocks. I’m going back to find them.’

‘I’ll go the other way,’ Edmund said. ‘It’s definitely lighter ahead, and there are glow-worms again – we mustn’t be far from the end of the tunnel. If I can get out that way, I can get back to camp for tools.’

‘What if this fills up before you get back?’ Tiffany shouted.

Which was exactly what they were all afraid of, and exactly what they didn’t want her to think.

There was a long moment of silence. Long enough to hear that the drops plinking into the water had become a steady stream.

‘We’ll get you out before it does,’ Tristan promised desperately.

Ollie started crying louder.

‘We’ll leave Tristan the backpacks,’ Nim said. ‘There might be something useful we’ve missed.’

Edmund took off his headlamp and gave that to Tristan too. Fred stopped hunting glow-worms and raced to Nim’s shoulder. ‘I’ll be back soon!’ she called. What she really wanted to say was, I’m sorry I brought you here! I’m sorry I found the fossil and I’m sorry you fell down the hole even though I didn’t know it was there, because it’s my mountain and my tunnel and so it’s my fault. But that wouldn’t make anything better right now. Right now they couldn’t waste time worrying: they needed to act.

Because if that shaft filled up with water before they got Tiffany out, she would drown.

Ollie’s crying grew to a wail.

‘Take Ollie with you,’ Tiffany shouted.

‘No! Staying with Tris! Want Tiff to get out now!’

Edmund hesitated. The stream of water grew heavier. Ollie’s wail grew to a scream.

Tristan stood up. Even in the darkness his expression looked torn. ‘I’ll go for help if Edmund will stay. Ollie, you’re coming with me.’ He handed the headlamp back to Edmund.

‘Wait!’ Nim shouted. ‘If you can get out that way, follow the trail beside the creek. Alex Rover’s studio is in the rainforest on the way to the house. She can use the satellite phone to call Jack. And she might have some ideas on how to get Tiffany out.’

‘I need real help, not a storybook hero!’ Tiffany shrieked.

‘Alex Rover’s here now?’ asked Edmund.

‘She’s real and she’s here,’ said Nim. ‘I’ll explain later.’

She was pretty sure Alex would agree that keeping her secret wasn’t as important as saving Tiffany.

There was only a narrow strip of floor around the edge of the hole. Nim waited to see Tristan and Ollie sidle safely around it, and then she started back up the tunnel the other way.

Edmund dumped out the backpacks, searching all over again for anything that could possibly help Tiffany get her foot out of the crack in the rock.

Tiffany stayed exactly where she was, listening to the water run down the sides of the shaft into the pool below, and wondering how fast it would rise. It was already up to her left ankle.

LEADING OLLIE BY the hand, Tristan felt his way down the tunnel. It was true that there was a tinge of grey ahead in the blackness, and the dancing blue lights of glowworms, but they weren’t enough to show him where the tunnel led, or if there was another giant hole ahead. So he slid his feet along the floor and patted his free hand along the walls, while part of him screamed, Hurry up and save your sister! and another part – one that didn’t want to die falling down a hole inside a mountain – screamed, Careful, go slow!

The floor was smooth, but not wet-slippery: the water was all going down the shaft where Tiffany was stuck.

‘But it’s probably leaking out somewhere,’ he told himself. ‘And the rain’s probably stopped now anyway.’

The tunnel curved and the slope became steeper. The roof was lower and the light brighter. Tristan put Ollie on his lap and skidded on his bottom like a kid on a slippery slide, around a curve and out into fresh air and rain.

The rain was coming down in bucketfuls. It splashed off rocks and turned dirt to mud. It gathered up Tristan and his little brother and swooshed them down the hill on the wildest, craziest ride of their lives.

They landed with a jolt on an arch of rock beside a waterfall. Tristan’s heart slowed its thumping, and he rolled to his knees to look over the edge.

Far below was the pond Edmund had taken a picture of yesterday. ‘It looks much prettier when you’re not falling into it!’ Nim had said.

So now there were two worst things that could happen. His little brother could fall off the cliff and drown. And his sister could stay stuck deep inside the cliff and drown.

Because the other thing Nim had said was that a waterfall came out of the tunnel when it rained. Rain was pouring into the start of the tunnel now, but it wasn’t flowing out this end yet. First it had to fill in every hole along the way. Including the one Tiffany was stuck in.

Tristan’s heart began pounding again. As if he was the one stuck in the shaft, he felt darkness surrounding him, the rock closing him in, his body crushed by pain and terror.

It took all his strength to break free of that link. But if he was going to save his sister, he had to save himself first. And he was stuck halfway down a cliff, on a narrow bridge of rock, with an exhausted toddler in his arms.

NIM CREPT STEADILY up the tunnel, trying to keep to the edges where it was drier and not so slippery. Her headlamp glowed dimly; the trickling water was the only sound.

‘I’m glad you’re with me, Fred,’ she said, as he rubbed his spiny back against her neck. Fred pressed a little closer. He loved sunshine and being able to move fast when his blood warmed up, but he loved Nim more.

‘We don’t need to be quiet,’ Nim reminded him. ‘It’s okay if Lance and Leonora hear us now.’

Fred sneezed.

‘Truly. They’ll help us when I tell them what’s happened.’

But her voice still came out in a whisper.

Besides, it was easier to be quiet so she could concentrate on climbing up the tunnel without slipping backwards or falling down any other holes. She didn’t know how long she’d been climbing, and she didn’t know how long it had taken them to go down in the first place. Now there was another tunnel shooting off from the side. She was sure she was still in the right one, but not one hundred per cent sure.

And she had no idea how close she was to the cave. Or what she’d do if Lance and Leonora had already gone.

Go to the Emergency Cave. There was rope there, and tools. Maybe she should have gone there in the first place. All she’d thought about was getting Tiffany and Ollie out right away.

She came to another side tunnel, and then another, and now she knew for sure that she was in the right tunnel and that it wasn’t far to the bats’ nursery cave.

She could hear voices: Leonora’s, and then Lance’s.

Nim remembered Leonora’s smile, and the way that she’d listened to everything Nim had to say last night. Everything was going to be all right.

‘Help!’ she started to call.

Then she heard what they were saying, and the ‘Help!’ slipped back down her throat as quickly as a chunk of coconut into Fred’s stomach.

‘Careful!’ said Leonora’s voice. ‘We don’t want it to blow up too soon!’

Blow up? thought Nim, creeping closer.

‘Don’t worry – I won’t light the fuse till we’re out.’

‘Are you sure that’s the best place?’

‘Perfect!’ said Lance. ‘When it collapses this cave it’ll blow out the wall between here and the opal. No more stinky bats to worry about – and you’ll have all the room you want to cut out that whole fossil.’

‘The most perfect fossilised sea turtle ever found! I’ll be famous!’

‘Or we could break it into bits and sell the opals. There are some valuable pieces in there.’

‘Rich or famous – we can’t lose!’ Leonora gloated.

Oh, yes you can! Nim wanted to shout.

‘Okay, it’s set!’ said Lance. ‘Let’s get out quick. Three minutes till BOOM time, and this cave is dust. Bye, bye, bats!’

‘Good riddance to the filthy things!’ Leonora said, her voice disappearing as she scurried out through the door hole.

They’re going to kill the bats! Nim’s mind screamed. And if I don’t get out of here, they’re going to kill me too.

Lance said three minutes, said a calmer part of her mind. All you have to do is cut the fuse. The dynamite can’t go off if the spark doesn’t get there.

‘Hang on, Fred,’ she whispered. Fred clung to her shoulder a little tighter, and she crept the last three steps into the nursery cave. Her headlamp swooped around, lighting up the sleeping bats, the dripping stalactites and the pillars of stalagmite, the green-glowing fungi and the blue swinging glow-worms. It wasn’t just a home to one of the world’s most endangered species, it was a magical, geological, biological wonderland. And it was part of her island. It was not going to be destroyed.

Nim stepped into the middle of the cave and finally saw the dynamite. It was beside the stalagmite at the entrance to the fossil tunnel. Six little sticks, tied in a bundle, with a long rope of fuse leading out of the cave.

They looked just like they did in pictures, except they were horribly, dangerously real: six little sticks of evil. She didn’t want to touch them.

She reached for her knife – and then she remembered. The pocketknife wasn’t around her neck. It was with Tiffany in a hole filling up with water.

Nim didn’t know that her brain could run so fast in so many different directions while her body was frozen in fear. She couldn’t run far enough down the tunnel in three minutes to be safe.

Lance and Leonora weren’t going to help her save Tiffany.

When the cave blew up the mother and baby bats in the nursery would all die and the bats would become extinct.

Alex Rover’s Hero would pick up the dynamite and throw it down the cliff.

If the bat cave exploded, the start of the tunnel would fill up with rocks.

Tiffany was going to drown unless she stopped the rainwater from flowing down the tunnel into the shaft.

The only way to stop the water was to block the tunnel. The only way to block the tunnel was with a rockfall. And the only way to have a rockfall in just the right place …

If she threw the dynamite out to save the bats she’d be killing Tiffany.