EVERYONE DONNED FLIPPERS AND MASKS. THEY ALSO wore flotation vests because Dr. Geaux and Calvin were sticklers about safety. The vests provided enough buoyancy to keep a swimmer on the surface of the water, but weren’t so bulky it made it hard to dive.

“Wouldn’t it be cool if we found the Black Thunder out there?” Stuke asked.

“Don’t be ridiculous, Stuke,” Riley said. “That’s just a story.”

“What’s Black Thunder?” Emmet asked. “Some kind of rogue shark?”

Raeburn laughed. “No. It’s a legend about a haunted pirate ship. Supposedly it sunk somewhere nearby, with a fortune in gold aboard. A notorious pirate named One Leg Sterling captained it. He had just raided a British man-of-war and taken on a lot of gold coins. His ship was damaged in the battle, though, and a sudden storm came up. The extra weight of all that gold caused it to sink, supposedly near here. Treasure hunters have been searching for the wreck for years. People say they sometimes see a pirate walking along the shore during the full moon — that it’s Sterling looking for his lost treasure. But it’s just a myth.” Everyone had put on their snorkeling gear, and Emmet stared at the water. Far offshore a fishing boat bobbed at anchor.

Emmet felt something on his shoulder and jumped. Luckily it was only Calvin, not a ghost pirate or a Pterogator.

“Are you all right, dude? You look a little pale,” Calvin said. He knew how skittish Emmet was about the water now.

“Don’t do that!” Emmet complained.

“Do what?” Calvin asked, seriously confused.

“Don’t sneak up on me like that. I could have gone all ninja on you,” Emmet said. In addition to visiting the library nearly every day for a thorough study of Florida’s fauna this summer, Emmet had insisted his dad enroll him in a karate class. Even though no one agreed with him, he still believed Dr. Catalyst was out there creating a giant opossum that knew kung fu. Emmet wanted to be ready.

“I wasn’t sneaking up on you, I was standing right here. And I didn’t know you’d made it all the way to ninja in two months of classes,” Calvin said. Emmet was reasonably sure he was joking, but you could never tell with Calvin.

“You sneaked. You definitely sneaked. And okay, I’m a little leery about going in the water,” Emmet said. “But I’ve got mad ninja skills now, so don’t test me.”

“What do you say we snorkel?” Calvin stalked off toward the water, walking awkwardly in his flippers.

“Okay. Snorkeling. It’ll be fun. Let’s follow Calvin. Nothing to worry about. Water’s not that deep and stuff. No problem,” Emmet said as he cautiously traipsed toward the water. The others seemed to walk easily into the ocean in their flippers, but Emmet lifted each foot and carefully stepped along like he was wearing giant clown shoes. At least it was warm. Warm water was probably better for sharks, though, and stingrays, giant squid, and killer whales.

“Hurry up,” Calvin hollered. “We need to stay together.”

Emmet quickened his pace and soon joined the others in waist-deep water. Everyone dived forward and swam out together.

Calvin was carrying a dive flag and swam a short distance ahead of them. A small weight was attached to the flag by rope, and he lowered it until it settled on the bottom. The bright red flag had a white stripe running at an angle across it. It was attached atop a small plastic float, which bobbed gently on the water. The flag would let any passing boaters know there were divers in the area. In Florida, it was illegal to snorkel or scuba dive without a dive flag. Calvin was a law-and-order guy.

Emmet was new to snorkeling but got the hang of it pretty quickly. It wasn’t really swimming. It was mostly floating until something interesting appeared, then diving for a closer look, and staying down for as long as possible while holding his breath. Upon surfacing he blew the water out of his snorkel. Emmet found he could float for a long time, breathing through the plastic tube and observing the sea life.

The five of them floated in a rough semicircle, looking down into the waters below them. Emmet had to admit it was pretty spectacular. The water was a deep sky-blue color, and there were lots of cool fish and plants to look at. Parts of the coral reef were covered with indentations on the surface that made him think of pictures of the moon he’d seen in science class. Other parts stretched out like vibrantly hued fingers reaching into the ocean, or silk fans that swayed lazily in the currents. Emmet couldn’t believe there were this many different colors under the water. Oranges, yellows, reds — and not just on the reef and the plants, but on the fish as well. It was a revelation to him, because he had never been in the ocean before. He understood now what Dr. Geaux had been trying to tell him. It was like nothing he’d ever seen.

He recognized a few of the fish swimming around, including a small school of groupers floating a few yards away near the surface. But around the reef itself there were hundreds of scary fish. They were reddish-orange with white stripes, and had sharp, dangerous-looking spines sticking out all over.

Emmet raised his head and tapped Raeburn on the shoulder. She looked up at him as they treaded water.

“What are those spiny fish? They look like they belong on the head of a medieval weapon,” he said.

“They’re lionfish,” she said, shaking her head.

Great! Emmet thought. In Florida even the fish are named after things that eat you.

“Why did you shake your head?” he asked, glancing around to see Stuke had floated away from the group and Riley was trying to get Calvin’s attention by pointing at something beneath the water.

“They’re nonnative. An invasive species, just like the pythons Dr. Catalyst was trying to get rid of,” she said. “They’re really aggressive and they sting, so don’t get too close.”

“Of course they sting. It’s probably fatal,” Emmet said.

Raeburn shook her head. “No, it just hurts. They chase off the other fish that feed around the reefs and then strip the reefs clean. They never recover.”

“Can’t they be caught or something? Like in nets?”

“They’re too deep for nets, and they’re hard to catch. Divers will sometimes spear them, and they can be eaten if you clean them right, but there are just too many of them now.”

“Huh,” Emmet said. “How are you so smart about all this stuff again?”

“It’s called studying, Emmet. You should try it sometime,” Raeburn said. She adjusted her mask and dived under the surface.

Emmet decided to follow her.