ALEX STARED OUT of her studio window. The rainforest was cool and green, but Alex was writing. All she saw was the hot dry desert that her Hero and his camel were trudging across, tired and thirsty …

‘That’s ridiculous!’ she scolded herself. ‘They’ll never get there in time.’

If her Hero didn’t find the treasure and stop the Bad Guys from blowing up the temple, that would be the end of the story, and it wasn’t the end Alex wanted.

Now the Hero was riding and the camel was galloping. Puffs of sand flew up from the camel’s hooves; the air was hot in the Hero’s face.

‘That’s better,’ said Alex. She watched him gallop through the narrow streets of the ancient city and leap off at the temple door.

‘But now the camel will die if the dynamite goes off!’

Worse, the temple and the square around it were crowded with people. The Bad Guys didn’t care – they just wanted to blow it up to get the treasure they thought was buried beneath it.

Her Hero had to be smart as well as brave. He had to make sure that everyone else was safe before he risked his own life to save a building.

He turned the camel around and whacked it on the rump. ‘Shoo!’ he shouted, waving his arms till the camel ran and everyone raced out to follow it to safety.

The Bad Guys were fleeing out the back of the temple. They’d set the dynamite just before the Hero got there.

He’s not going to be searching for treasure when the building’s about to blow up, Alex thought. But if he saves the temple, maybe he somehow uses the dynamite to catch the Bad Guys – and that’s when he realises he’s found the treasure. Right now he doesn’t have time for anything except finding the dynamite and making a lightning decision about what to do.

Which was why she had to work out how much time he needed.

If the temple is ten times as big as my studio, then the treasure would be hidden under that tree near the path… I’ll time how long it takes Nim to run from the tree to the door. Running through the forest will be like running around all the statues and things in the temple.

But they couldn’t do the test today because of the visitors. That reminded her, there was something she was going to look up, something to do with Nim and whatever she’d been doing this morning. Because as soon as she’d seen her coming back down the trail so early, Alex knew Nim was up to something. She just didn’t know what.

She shook her head. She still couldn’t remember what she’d meant to look up – it mustn’t have been important.

Alex turned over the page, and started sketching a plan of the temple.

EDMUND WAS TELLING Nim about his school. Nim had read lots of books about schools, but it was different when Edmund told it. She’d never really imagined what it would be like to sit at a desk for a whole hour. ‘But don’t you ever feel like you’re going to burst if you don’t get up and run around?’

Edmund laughed. ‘All the time,’ he said, so even though she wasn’t quite sure why he was laughing, she knew that he wasn’t laughing at her. It was a whole lot easier being friends when they were alone.

‘Wait for me!’

Tristan was jogging across the grasslands behind them. They stopped for him to catch up.

‘Are Tiffany and Ollie coming too?’ asked Nim.

Tristan shook his head. ‘I don’t have to do everything Tiff says.’

Nim led them through the rainforest, wide around the Hissing Stones, where the yellow pool splattered its slimy bubbles and steamy water fountained high. They couldn’t go wide enough to miss the stink. It smelled as if a hundred rotten eggs had all smashed at once.

Tristan held his nose. Edmund looked as if he wanted to. Fred sneezed. They were all glad to get to the Black Rocks.

The boys weren’t as used to rock climbing as Nim; Edmund slipped off one rock and skidded down another before he got his balance.

‘Sorry, Fred,’ he said, patting the spiny head.

Fred rubbed against his neck. Fred liked sliding.

At the top of the cliff they stopped. They could see far over the sea, and over the rocks they’d just climbed, and south to the rainforest that Nim had climbed that morning.

Below was the deep sea, where Nim and Fred had dived to save Selkie from the Troppo Tourists. Further along, the sharp rocks formed a small cove, but it was not a calm and peaceful place like Keyhole Cove. This was the island’s wild side, and it could never quite be trusted.

They climbed on up the path to the door hole of Nim’s new cave.

‘I thought you said it was big!’ said Edmund.

‘Just wait till you get inside,’ said Nim.

She put on her headlamp and slid in through the hole. She slid a bit faster than she meant to, because of showing the boys how, and landed harder than her knees wanted. She spat on her hands and rubbed the blood off before anyone saw.

Tristan followed, pulling a mini torch out of his pocket. Edmund put on a headlamp and slid through last, with Fred still on his shoulder.

Nim stopped in the glowing cavern for them to catch up. Tristan was staring around in awe: he bumped into her, then Edmund bumped into him, and then Fred got frightened and jumped from Edmund’s shoulder to Tristan’s and across to Nim.

‘Ouch!’ said Tristan. Fred’s claws weren’t actually sharper when he was frightened, but he dug them in deeper.

‘Don’t disturb the bats!’ Nim shushed, rubbing Fred’s head.

She slipped around the big stalagmite into the side tunnel. Her headlamp lit up the leaf pattern in the rock.

Edmund shrugged his daypack off his back and pulled out a paintbrush.

‘You can’t paint it!’ Nim hissed.

‘I can do something better,’ said Edmund.

Very gently, he began brushing the wall around the fossil. Whitish dust drifted away, and the pattern stood out as clearly as if it had just been pressed into clay. It wasn’t a fern – it was seaweed.

‘Cool!’ said Tristan. ‘What’s that other thing?’

On the wall where Edmund had started brushing, a spark of blue danced in the torchlight.

‘Oh,’ Nim said sadly. She ran her fingers over the bulge that she’d thought might be a fossilised branch. But she was very sure that fossils weren’t blue, and just as sure that something this big couldn’t be anything to do with the delicate sea plant.

Tristan rubbed with his hand, and Edmund started brushing again. More blue appeared, with sparks of fire dancing inside it.

‘It’s an opal!’ Edmund exclaimed.

They all ran their hands over the wall, feeling from the ridge where the blue started, up, down and along.

‘It’s huge!’ Tristan breathed.

Nim was still disappointed that it wasn’t a fossil, but the more of the fiery blue she saw, the more beautiful it seemed. She got out her chisel and chipped gently away at the rock above where whatever-it-was began.

Edmund got a smaller brush out of his daypack and gave it to Tristan.

‘I’ve got a comb too,’ said Tristan, and they all laughed. But even as they joked about brushing and combing the rock’s hair, they kept on doing it. The shape stood out more and more clearly as the boys brushed away the flakes of rock that fell from Nim’s chisel. Then Tristan ran his comb over the bulge itself, and when Edmund brushed those dirt crumbs away, more sparks of blue and green glowed from the wall.

They went on working. When Nim had chiselled all around the outside of the shape, she used the spike on her pocketknife to pry the last bits of limestone away from the edges. The boys went on brushing. The opal grew and grew.

‘It’s like finding buried treasure!’ Tristan exclaimed.

After a while they stopped to eat the coconut and bananas, and an energy bar Tristan had in his pocket. Then they went on uncovering whatever was on the wall.

It was a round shape, longer than Nim could spread her arms wide, and curved out gently in the middle.

‘Here’s something else,’ said Edmund, brushing just below it.

Very delicately, Nim picked around it with her spike. Edmund flicked the crumbs off, brushing and brushing until a long, thin piece of opal appeared.

‘It looks like a bone,’ said Tristan.

Edmund brushed a bit more. The blue-green, sparking-with-red opal was long and thin and knobbly.

‘Like a tailbone,’ said Nim.

Edmund brushed more carefully still, flicking out the grains of dirt around the knobbles.

Nim went back to the big shape. She’d uncovered the outside nearly all the way around now: an oval ring of hard, shining stone in the softer rock of the wall. There was a wide band across the middle, and ribs of colour stretching across from the band to the outer edge. She ran her hands over the whole thing again.

It was the same shape as Chica’s shell.

Nim stood back and let the light of her headlamp rove slowly over the wall. ‘It’s a sea turtle,’ she said at last.

‘Fossilised into opal,’ Edmund said slowly, still brushing.

‘And not just the shell,’ said Tristan. ‘I’ve found its head.’

They all stood back and stared. The shape Tristan was uncovering was oblong … exactly like a skull. At the other end of the shell, Edmund had just brushed away the dirt from a longer bone, with a ray of smaller bones sprouting from the end.

‘Sea turtles don’t have fingers, do they?’ Edmund asked.

‘No,’ said Nim. ‘But Jack says the bones inside their flippers are pretty much the same as our feet and toes.’

They all stared at it for a long time. ‘Wow,’ Tristan said at last. ‘That’s the coolest thing I’ve ever seen.’

‘I was hoping it was going to be a fern tree branch,’ Nim admitted.

‘That’s too bad,’ said Edmund, sounding so serious that for a moment Nim was tricked. ‘You wanted a bit of fern and instead you got a whole opal fossil sea turtle. Probably the rarest fossil in the whole world.’

Tristan laughed, and that made Edmund start and then Nim. Even when they got out their water bottles they kept spluttering and leaking water out of their mouths while they drank.

Finally Edmund pulled out his camera and started taking pictures. Tristan and Nim shone the lights on the fossil so the glowing opal flashed sparks of fire against the darkness.

‘Chica’s great-great – millions of great – grandmother,’ Nim said softly. ‘Now she’s like a secret jewel inside the mountain.’

‘She is a jewel,’ said Edmund.

LEONORA AND LANCE had headed off towards Frigate Bird Cliffs, just like they said. But they hadn’t said how far they were going, and they didn’t go very far at all. At the first hollow, they sat down to wait. Every few minutes one of them would lie on their stomach and peek over the rise to spy on the camp with Lance’s binoculars.

Because Leonora had been studying Nim ever since they arrived on the island, and she was sure that Nim was hiding something. Especially this morning when she’d heard bits of whispered words: fossil … amazing … cave.

So what they wanted to explore was whatever Nim didn’t want them to see.

They watched Selkie go down to the ocean and Nim and Edmund head across the grasslands.

‘The twins have stayed behind!’ Lance said in disgust. ‘That’s going to complicate things.’

‘They’re just kids,’ said Leonora. ‘We won’t have any trouble outsmarting them.’

They thought for a few minutes. ‘If it’s part of the cave we went to yesterday, we can go up the hill and around so the twins don’t see us.’

‘But we’ll be able to catch up so we can see exactly where Nim is going.’

Lance poked his binoculars over the lip of the hollow again. The camp appeared empty, but he glimpsed the back of a dark head as Tristan disappeared around a grassy hill.

‘Even better!’ he laughed. ‘One thing’s for sure, those twins stick together. If you see one, you know the other’s there too.’

‘And with five kids to follow, it’s going to be that much easier to spot one of them …’

‘ … and let them lead us to whatever it is they’ve found.’

They smiled smugly as they climbed out of their hollow. They couldn’t see the group but it didn’t matter now they’d guessed where they were heading. Once, when they pulled out their binoculars, they saw Nim starting to climb the Black Rocks. They waited out of sight in the rainforest until they guessed everyone was at the top.

By the time they’d climbed the rocks too, they were just in time to see Edmund’s legs sliding into a hole in the side of the cliff. ‘That was lucky!’ said Lance.

‘I told you,’ said Leonora.

‘I’ve got a very good feeling about this.’ They waited a few more minutes, and then crept carefully up the path to the cave. For a long, long time, they sat outside, taking turns to stick their heads through the door hole and listen.