IT WAS STILL dark when Nim slipped her headlamp onto her forehead and tiptoed out of the house. Fred scuttled out from his rock, and Selkie slid across the porch, whuffling and snorting hello. They never cared how early it was; if Nim was doing something, they wanted to do it with her.

Fred especially loved it when Nim was wearing her headlamp, because he could do his two favourite things at the same time: riding on Nim’s shoulder and catching the insects that flew into the light.

Selkie lollopped ahead. Selkie was a lot bigger than Nim, but she could move nearly as quietly, and much faster. She started down the hill as Nim filled her bamboo drinker from the waterfall.

‘Wait!’ Nim called. ‘We’re going to the cave.’

Selkie snorted a disapproving sort of humph. ‘Not the Emergency Cave,’ Nim added quickly. ‘I think I’ve found a new one.’

Selkie humphed again. She didn’t care which cave it was: she didn’t like any of the caverns and tunnels on the cliffs above the Black Rocks.

‘We’ll meet you there,’ said Nim.

Selkie sighed a deep, sea lion sigh, and galumphed on down the hill to the sea. Nim and Fred followed the creek deeper into the rainforest.

They passed the side-trail to Alex Rover’s writing studio. Alex liked having her studio away from the house so that she had to go outside twice a day, because when she was in the middle of writing a book, she forgot about things like going for a walk, or even eating. Sometimes Nim had to go and knock on the door and remind her that it was time for dinner, just like she had to remind Jack when he was busy with science experiments.

Luckily Nim always had Selkie and Fred to remind her when it was time to eat. They never forgot.

After Alex’s studio, the rocks beside the creek got bigger. The trail got skinnier and the rainforest got thicker. Vines dangled down from the trees and across the ground, ghostly and shadowed in the bobbing light of Nim’s headlamp. It was hard to tell if they were vines or snakes.

Nim liked watching snakes, and she liked stroking their sun-warmed, gleaming skin, but she didn’t like accidentally bumping into them. They mightn’t mean to hurt, but if they were frightened and bit her, she could die anyway.

It was hard to believe that something so beautiful could kill you.

Fred believed it. Every big vine reminded him of the python that had wanted to eat him for lunch. Pythons aren’t venomous, but you’ll still end up dead if they swallow you. Nim had grabbed Fred just in time, and they’d both felt shivery for a long time.

He snuggled close into her neck now. Nim tickled his chin.

‘Don’t worry,’ she said. ‘I’ll never let anyone hurt you.’ Fred sneezed gratefully. He was glad when the blackness faded to a grey dawn light, and it was easier to see if the snakes lying across the trail were real hissing, biting ones, or just vines pretending.

Now Nim could hear the waterfall rumbling off the cliffs, and the creek rushing as the water spilled into it from the pond at the bottom.

Through the shadows, she could see the dark shape of the rock bridge arching over the pond to the cliff.

The cliff looked high from here, but it had seemed even higher when she’d climbed it with Edmund. Especially when he’d fallen off into the pond, and she’d jumped in after him. That was what they hadn’t wanted to tell Tristan and Tiffany yesterday.

But falling into the pond was how they’d found the cave at the bottom of the cliff – and the bats. They were big brown fruit bats, and there was only one other colony of this species in the whole world. They were just about as rare as anything could be without being extinct.

The bats were one of the reasons that the World Organisation of Scientists had classified Nim’s island as a wildlife sanctuary. It was like a circle, Nim thought: finding the bats had helped keep her island safe, because the world wanted the island to keep the bats safe.

Then the sun rose behind Fire Mountain. Nim turned off her headlamp; Fred swallowed one last bug and fell asleep on her shoulder. Pink light trickled through the trees, glinting off the waterfall, pond and creek. But Nim hurried on the other way, across the creek and around the side of the hill. It was time to find out if she really had discovered another new cave.

SELKIE WAS WAITING at the top of the Black Rocks, whuffling reproachfully.

‘Sorry,’ said Nim, ‘but this will be worth it.’

They followed the path to the tree where she’d felt the cool breeze, and there it was: a hole in the rock, like a door into adventure. It was about as high as her waist: Nim bent and looked into the dark nothingness. She switched on her headlamp.

NO! Selkie barked, in her deepest, strongest voice, because the hole was big enough for Nim and even Jack, but not for a sea lion. Selkie didn’t like Nim going where she couldn’t follow.

For a moment Nim wished she’d woken Edmund up to come with her. It would have been fun to have another adventure together – and Selkie was right about how dangerous it was to go into a cave on her own. But she’d come all the way up here, she wasn’t going to turn around now.

‘I just want to have a look,’ she said. ‘I won’t be long.’

She stretched further into the hole until she could touch the ground on the other side, and slid into the cave.

Except that it wasn’t a cave after all, just a tunnel: she couldn’t quite stand up in it.

‘I thought it would be big!’ she told Fred.

Fred didn’t answer. Selkie stuck her head in and barked that if Nim didn’t know what it was like she should come straight back out.

But Nim could already see that the tunnel got steadily wider and higher, so she crouch-walked, and then she walked standing straight, and then her headlamp was shining into the biggest cave she had ever imagined. It was as tall as a building in Alex Rover’s city and as magical as something in one of her stories. Because it wasn’t just big: it was glimmering with light.

Nim turned her headlamp off and stared.

Strings of shimmering blue glow-worms danced from the ceiling, their tiny lights turning rocks into jewels. Toadstools glimmered green from the floor: fat and lumpy or delicate and spindly, but all shining with a soft, eerie glow. Stalactites dripped down and stalagmites grew up. Some were as small as her little finger but the oldest were as tall and thick as trees; in some places they’d joined to make solid pillars from roof to ceiling. There were arches that looked like doorways to more tunnels and mysteries, and windows into blackness.

In the centre of the cavern, big brown fruit bats were circling and squeaking.

What are they doing here? thought Nim.

Because they were definitely the same type of bats that lived in the cave under the Waterfall Cliffs, and they were definitely settling down here. Except they’re settling up, she thought, watching the bats grab their favourite ceiling spots with their toes, so they could dangle upside down for a good day’s sleep.

She went nearer. Each of the upside-down bats had a tiny baby bat clinging to her stomach.

‘It’s the nursery cave!’ Nim exclaimed. She’d read lots about bats after she and Edmund had found the main colony. One of her favourite facts was that some species had a separate place for the mothers and babies until the pups were old enough to live in the main cave. Jack didn’t think their bats did that, but Nim had always hoped so. She liked thinking about all those mothers and babies in a special safe place.

And here it was: the nursery for the most endangered bats in the world.

The only mystery was why she hadn’t seen any flying in when she was outside the entrance. She hadn’t even seen any droppings in the tunnel.

If the bats were being so careful to keep this nursery hidden, it was even more important not to disturb them. Nim tiptoed back to where she’d come in. It was easy to find because a huge stalagmite, as fat around as Selkie, stood just by the entrance to the tunnel.

Another tunnel led off to the left. It was tall enough to walk in and curved like a rainbow: ‘So it can’t go far,’ Nim told Fred, and followed it.

But the tunnel didn’t go anywhere at all; around the curve it widened and ended against a wall of rock. ‘We must be right next to where we came in,’ said Nim. ‘Can you hear us, Selkie?’

Selkie barked yes. Then in case they hadn’t heard the first time, she stuck her head as far as she could into the entrance and barked again. The rumble was so loud that the end wall shook. Crumbs of limestone drifted down.

‘We’re just on the other side from you!’ Nim shouted back, knocking hard to make sure Selkie understood. A whole flake of rock crumbled and fell.

‘Oops!’ said Nim.

Then she saw what the flake had been hiding. The pattern of a fern stood out clearly where the piece had fallen away.

She touched it. The shape felt as if it had been carved, but Nim knew that wasn’t true. This fern had been a living leaf once upon a time, until it had been covered with mud, and slowly, over thousands and millions of years, it had turned into a fossil.

Her fingers tingled, knowing she was touching something that had lived so long ago. It wasn’t very big, but maybe there was another one beside it – maybe even a bigger one. She could bring Edmund back up here to help her search. He couldn’t always keep quiet enough to find live animals, but he couldn’t scare off a dead plant.

And when they showed everyone else … She imagined Leonora saying, ‘I’ve never seen such a perfect fossil! That’s exactly the clue we need to find a biofuel.’

A hand’s breadth away from the fern there was a bulge in the rock. Nim ran her fingers over it. She could feel an edge, and the more she rubbed, the more she could feel.

‘It’s a branch,’ she told Fred. ‘It must be a giant fern. I wonder if I’m the first person who ever found a giant fern fossil?’

Fred didn’t answer, even with a sneeze. The cave was cold, and Fred was cold blooded: it was getting hard for him to move.

‘Sorry, Fred,’ said Nim. ‘Time for you to soak up some sunshine.’ She rubbed the edge of the bulge one last time, and a drift of soft limestone crumbled away. The branch was even bigger than she’d thought.

Nim forgot that Fred was cold and Selkie was worried. She rubbed around the edge. She spat the dirt off her hands and rubbed again.

More rock crumbled away. The bulge bulged bigger. It was more like a log than a branch.

Selkie barked again. If you don’t come out now, I’ll come in! that bark said.

Nim left her fossil and raced back around the stalagmite. Finding her way out would be easy, because a bit of daylight should be coming down the tunnel from the entrance.

There was no daylight; no way out.

I’m trapped! Nim thought – just for a second, but it was a long second. And then she realised: there was no daylight because the door hole was completely blocked by a sea lion’s head and shoulders.

‘Go back, Selkie!’ Nim shouted. If Selkie got stuck, they’d all be here forever, until they turned into fossils too.

Selkie humphed, backed up – and finally there was light. Nim slid through the hole and lay on the path, blinking in the bright sunshine. The sea lion sniffed and checked her. Fred gave a pathetic snort of sneeze.

But they had no time to waste. Bright sunshine meant it was time to get back to the house, and then to the camp.