I’m back.

That’s what I thought when I arrived on the Cassandra for another summer vacation.

Yes, I, William Deep, Jr., world-famous undersea explorer, am back.

One year older. One year wiser. One year tougher.

I breathed in a big gulp of salty air. I gazed at the clear green Caribbean sea around me.

My little sister, Sheena, stood beside me. But I pretended she wasn’t there.

She sort of ruined the atmosphere for me. She usually does.

The Cassandra is my uncle’s boat. It’s a floating research lab.

My uncle, Dr. George Deep, is a marine biologist. My parents sent me and Sheena to visit him on our summer vacation. They sent us the year before too.

Dr. D. lives on the boat in the Caribbean all year long, studying tropical fish. It’s fun for us. We get to swim and stuff. My uncle is really nice. And my parents figure we’ll learn a lot about science and ocean life while we’re with him.

Last summer I made one of the most shocking discoveries in the history of marine biology.

I found a mermaid. A real mermaid.

No one believed me, of course. I wasn’t a grownup scientist. I was a twelve-year-old boy on vacation in the Caribbean.

Know-it-all Sheena thought I was lying.

My uncle, Dr. D., thought I was making it up. He didn’t believe in mermaids.

Until I proved him wrong.

We didn’t tell anyone about the mermaids. Some really bad guys wanted to capture them and put them in cages. To protect the mermaids, Sheena, Dr. D., and I agreed to keep them a secret.

So the world will never know…

And now, I’m back! I told myself. Billy Deep, one of the greatest explorers in the seven seas.

And I’m not a twelve-year-old kid anymore.

I’m thirteen.

And this summer, I’m going to find something big. Something even more amazing than a mermaid.

This time, the world will hear about it. This time I’ll be famous.

I hope.

* * *

The fire coral glowed bright red. I snorkeled near it, careful not to touch it.

I’d stepped on fire coral once before. It burned my foot like crazy.

They don’t call it fire coral for nothing.

I studied the coral wall. Neon-bright fish darted in and out of the delicate holes. It was beautiful. There, under the water, everything seemed calm. Quiet. Peaceful.

But I knew better. I was an experienced snorkeler. A snorkeling hero.

An untrained swimmer wouldn’t have noticed it. That little ripple in the water. The way the fish all suddenly disappeared.

But I felt it. That whiff of danger.

Something was coming. Something deadly.