She frowned.

"Althea and Marina will be so impressed. No one else has dared to do it." I hoped that would be enough to make her want to do it. I wasn't going to tell her it was so much more than that for me, that I was doing this to prove I was a real mermaid - not just to them, but to her, too. Before she had a chance to argue, I'd slithered into the slimy, echoey darkness. Eventually I heard her follow behind.

The winding tunnel led us deeper and deeper into dead rock: tight, cold, and claustrophobic, but gradually widening and growing brighter as we swam. Bit by bit, a growing circle of light opened up ahead of us.

We swam toward it, finally coming out into a dome-shaped s.p.a.ce in the middle of the cave. A high ceiling rippled faintly with the water's reflection.

"I don't understand," Shona said, looking around. "What's that light?"

I shook my head as we swam all around the rocky edges. It seemed to be coming from under the water.

"Come on." I dived down. "That's our answer!" I gasped. The floor of the cave was absolutely littered with crystals and stones and gold, all shining so brightly I almost had to shield my eyes. I'd never seen jewels like these. Dazzling pink rocks with sharp white edges lay on the ground in a circle, joined together by a thin line of gold. In their center, a bright blue stone shaped like a rocket pointed up to the surface of the water.

"What in the ocean . . . ?" Shona swam around and around the display, her mouth open, her eyes huge, shining with the reflection of the blue stone.

I looked around. There was more. Once we started looking, it seemed that stones and crystals covered the entire floor of the cave, packed and tucked into gaps in the rock all around us.

"Emily, I think we need to get back." A fat green angelfish hovered between us, its startled eyes staring into ours before it spun around and disappeared into a rocky crevice. "We've seen it now. We're not supposed to be here."

I stopped gazing around. Shona was right. "Okay," I said. "Let's go back." We'd found the answer to Althea's and Marina's questions. The lagoon hid a cave filled with jewels. But why? It didn't make sense.

Shona turned immediately and started making her way back toward the tunnel. But then I noticed something on the cave's wall: a picture exactly like the engraving we'd seen earlier, only even bigger. It looked like a mosaic. I knew that shape - I was sure of it. And even though it didn't make any sense, I had this overwhelming feeling that it knew me too! As we got closer, I could see it was made out of jewels: a huge golden one in the center, oval shaped and about half as tall as me, with multicolored strands spinning outward from it. I put my hand out to touch it. It wobbled.

"Shona!"

"Come on." She kept swimming.

I pushed at the jewel. It was lodged in the rock, but only loosely. We could probably get it out. I had to try. There was a secret in here - I was sure of it. Something was drawing me on and I couldn't resist it.

"Shona!" I called again. "Just look at this."

She stopped swimming and turned.

"It's loose!" I pulled at it, edging my fingertips underneath to lever it up. "Help me."

She swam reluctantly back to me. "I thought we were - sharks alive!"

"You thought we were sharks?"

Shona stared at the mosaic. "What is it?"

"Help me get it out."

"You're pulling my tail, aren't you? We can't go around vandalizing the place!"

"We're not vandalizing anything. We'll put it back. Let's just see what's behind it." An image of Althea's and Marina's faces flickered across my mind, their eyes wide and impressed with my bravery. All the other mermaids crowding around me, wanting to be my friends, accepting me as one of them, not the odd one out, not the freak. This cave was going to change my life; I just knew it.

Shona sighed heavily, then reluctantly dug her fingers under the jewel, and we gradually levered it little by little out of its hole. A moment later, we were holding it between us. We lowered it to the ground and it plopped onto the seabed with a soft thunk, scattering a shower of sand in a swirl around us.

"Now what?" Shona stared down at it.

I swam up to the hole it had left behind and poked my head into it. Another tunnel. I grabbed Shona's arm, pointing into the blackness. "We have to go down there."

"We don't have to go anywhere!" Shona snapped.

"Please! Aren't you dying to know what's in there? Can't you feel it?" This wasn't even about Althea and Marina anymore. It was more like a thirst, or a magnet pulling me.

A magnet? My throat closed up as I remembered. . . . But it couldn't lead to the Triangle. We were in the middle of the island.

Shona peered into the tunnel. Her eyes sparkled against the reflection of the crystals. I could see the dilemma in them. "We just have a quick look, see what's there, and then we go home," she said eventually.

"Deal!"

We edged our way carefully into the hole, slithering along in the silent dark. Me first, Shona following closely behind. The tunnel grew colder as we made our way deeper into the rock. The edges became craggy and sharp.

And then, without warning, it suddenly stopped. A dead end.

"Now what?" I called to Shona.

"We go back. We've looked. There's nothing here. And I'm not exactly surprised, or disappointed. Come on."

How could it suddenly end like that? I was sure it was leading somewhere. I felt around on the rock in front of me. It was different from the walls. Smoother. I inched my hands around it. Then I realized why it was different.

"Shona! It's a boulder!"

"What?"

"There's something blocking the tunnel. Look, it's different from the walls. Feel it."

Shona squeezed forward to touch the boulder.

I felt my way around its edges. "There's a crack all around it." It was almost the same shape as the crystal at the other end. "Maybe it'll come loose."

Shona looked at me.

"Let's just try."

"How do I let you talk me into all these things?" she said with another sigh.

"Because you can feel it, too? Because there's something down here that's making you tingle with excitement? Because the last time we went exploring, we ended up finding my dad? Because being my friend means you get to live on a beautiful desert island? Because -"

"Okay, enough." Shona half frowned, half smiled. "Don't get your tail in a tizzy. Let's just get on with it."

Because I couldn't turn back now if I wanted to, even if I don't know why. I didn't say that part out loud, though.

It didn't just slip out like the jewel at the other end. We pushed and pushed, but nothing happened. Or nearly nothing. The boulder moved slightly, rocking backward and forward as though it was on a hinge, but we couldn't shift it.

"It's useless," Shona gasped. "We'll never get it out."

"We need to use the rocking. Get a momentum going. Look, it's swaying. If we both push it from above, it might topple. Wait till I say. On the count of three. You ready?"

Shona nodded without looking at me.

"One." I felt around for a good hold on the rock.

"Two." I stretched out my tail, getting ready to flick it as hard as I could.

"Three!"

We swished and pushed, grunting and heaving.

"Now, let go!" The rock swayed away from us, and then back. "And again." Another shove against the rock, another slight movement. Again and again, we heaved and pushed until, finally, it started to loosen.

Then Shona stopped pushing. "I've had enough. I'm exhausted."

"But we're nearly there!"

"I want to go back," she said. "I don't want to do this."

"What's the problem?"

"The problem is that we don't know what's on the other side!"

"Exactly! But there is something, isn't there? I can almost feel it vibrating in my body."

"Me too. And I don't like it, Em. It doesn't feel good. I don't want to know what it is, and I want to go before this place collapses in on us."

"It's just a boulder. It's not going to collapse!"

But Shona turned to go back.

"Just one more push."

"You do it if you like. I'm going."

"Fine!" I went back to the boulder. It was teetering on the edge of the hole now. I could probably push it on my own. I didn't even know why I was doing it anymore. I just knew there was something here. I could feel it. Low vibrations hummed rhythmically through the cave, and inside me, growing stronger. What were they?

Fueled by frustration, I spun my tail as fast as I could, pushed all my weight against the rock, and heaved.

Very slowly, it teetered, swaying with the rhythm of the water before eventually toppling: a huge, smooth, oval rock slipping down and away from us, almost in slow motion. Water swirled all around. The boulder was still traveling - rolling, hurtling down through the water.

It felt like when you roll a s...o...b..ll down a hill and it grows bigger and bigger. Something was building up on the other side of the tunnel, below us, below the island, deep inside the rock. Nerve endings jangled and jammed like simmering explosions under my skin.

"I told you, I told you!" Shona screamed. "It's caving in! We're going to be trapped!"

"It's okay. Look." I tried to keep my nerve. Everything was still intact in the tunnel. It was just on the other side that the water was foaming and swirling everywhere. And there was something else: a presence. The vibrations had turned into a low rumbling, way down below. Something was down there. Something that didn't feel quite so exciting anymore. What was it? Images swirled around my mind: the mosaic, the spiral, whirling, spreading out, writhing.

"What's happening?" Shona screamed.

"It's just - it's the rock falling to the bottom of the caves," I said, much more confidently than I felt. "It's all right. Just stay calm. It'll stop in a minute."

The rock kept plummeting and crashing, getting fainter and fainter. Sand and rock particles swirled around, a few of them spinning softly through the hole into the cave.

And then it stopped. No more crashing. No swirling rocks or sand, no hurtling anywhere. Complete silence.

Total silence. Kind of eerie silence.

I smiled nervously at Shona. "See," I said. "Told you it'd all be okay."

And then we heard it. The rumbling. Not a flutter of excitement in our stomachs, or a thrilling vibration that we might have imagined. This was very, very real. And it was growing. Soon, a roaring noise sliced through the caves, growling louder and louder, rumbling toward us. I couldn't move. I looked at Shona. Her lips were moving - but I couldn't hear a thing. The rumble turned into a high-pitched whine, shrieking and screaming through the hole into the tunnel. I slammed my hands over my ears.

The next thing I knew, Shona had grabbed one of my hands. She pulled it away from my head. "We have to get OUT OF HERE!" she was yelling in my ear. "QUICK!"

I'd forgotten how to move. My tail, my arms, everything had turned completely stiff.

"Come on!" Shona yanked my arm, pulling me with her. My body jackknifed into action and we hammered through the tunnel as an explosion erupted in the water behind us.

I turned around to see the end of the tunnel crumble and dissolve. Rocks fell and bounced in the water, scattering sand and bubbles everywhere, clouds bunching and spilling across the seabed like lava.

Something was reaching out from the tunnel, feeling around. Oh, G.o.d! What was it? A huge tube, slimy and dark green, almost as thick as the tunnel itself. One side was rubbery and shiny, then it flipped and twisted over and its underside was gray and covered in black spots. They looked like giant warts. In between them, great thick suckers grabbed onto the wall like the suction cups on the soap holder Mom keeps in the shower, only about fifty times bigger - and a hundred times uglier.

That was it! That was what I saw in my mind only moments ago - and now it was here, for real, in front of me!

The tube flapped and flicked about, maniacally batting and thwacking against the sides, reaching out farther and farther toward us. An icy stake of terror pinned me to the spot.

The siren noise shrieked into the cave again as the tube thing moved around in the tunnel, feeling its way along. Getting closer!

Someone was screaming and screaming.

Shona shook me. "Emily, you have to pull yourself together!" she yelled. The screaming stopped. It had been me. She pulled me through the water. "Just swim for your life!"

We threw ourselves along the tunnel, working our arms like windmills in a tornado. I tore brief glances behind us as we swam. The tube lashed out, extending toward us like a giant worm, ripping at the tunnel walls and doubling my panic.

Propelling myself faster than I had ever swum in my life, I flung my body through the pa.s.sage until I finally made it to the open s.p.a.ce. The rock was collapsing around us as we swam.

The thing was reaching out of the tunnel toward us! No! Its end was tapered and blood red, and covered with brown hairy strands swirling around as it felt its way through the tunnel. It slid farther and farther out as we dashed across the cave to the next tunnel, the one that would get us out of here.

CRASH! THWACK! Slamming against the roof of the cave, the walls, the ground, the monster worm was destroying the cave, little by little. We were almost within its reach. Swim! Swim! Faster!

As we heaved ourselves into the next tunnel, I glanced behind me again. The giant worm wasn't on its own. There were at least three others, maybe more, all searching and feeling around the cave walls, crashing through the water, reaching out toward us. Slimy, scaly tentacles. What was it? A giant octopus?

A scream burned silently in my throat. Shona had virtually disappeared. She was ahead of me, but the water was murky with swirling pieces of rock and debris. One more corner, one more corner, I repeated to myself again and again as I plowed down the long narrow tunnel.

I threw myself at the end of the tunnel. Nearly out! I was panting and gasping, my energy slipping away. And then a tentacle spun out, coiling itself down the tunnel. It touched me! Arrggghhh! Rubbery slime grazed my arm. My speed instantly tripled.

A moment later, I was out. Out of the tunnel! Back outside in the channel between the cliffs. Sunlight.

Shona was there, panting and heaving.