“Annabeth,” I said. “She liked architecture.”

“She was nuts about monuments,” Thalia said.

“Spouted facts all the time.” Grover sniffled. “So annoying.”

“I wish she were here,” I said.

The others nodded. Zoë was still looking at us strangely, but I didn’t care. It seemed like cruel fate that we’d come to Hoover Dam, one of Annabeth’s personal favorites, and she wasn’t here to see it.

“We should go up there,” I said. “For her sake. Just to say we’ve been.”

“You are mad,” Zoë decided. “But that’s where the road is.” She pointed to a huge parking garage next to the top of the dam. “And so, sightseeing it is.”

We had to walk for almost an hour before we found a path that led up to the road. It came up on the east side of the river. Then we straggled back toward the dam. It was cold and windy on top. On one side, a big lake spread out, ringed by barren desert mountains. On the other side, the dam dropped away like the world’s most dangerous skateboard ramp, down to the river seven hundred feet below, and water that churned from the dam’s vents.

Thalia walked in the middle of the road, far away from the edges. Grover kept sniffing the wind and looking nervous. He didn’t say anything, but I knew he smelled monsters.

“How close are they?” I asked him.

He shook his head. “Maybe not close. The wind on the dam, the desert all around us . . . the scent can probably carry for miles. But it’s coming from several directions. I don’t like that.”

I didn’t either. It was already Wednesday, only two days until winter solstice, and we still had a long way to go. We didn’t need any more monsters.

“There’s a snack bar in the visitor center,” Thalia said.

“You’ve been here before?” I asked.

“Once. To see the guardians.” She pointed to the far end of the dam. Carved into the side of the cliff was a little plaza with two big bronze statues. They looked kind of like Oscar statues with wings.

“They were dedicated to Zeus when the dam was built,” Thalia said. “A gift from Athena.”

Tourists were clustered all around them. They seemed to be looking at the statues’ feet.

“What are they doing?” I asked.

“Rubbing the toes,” Thalia said. “They think it’s good luck.”

“Why?”

She shook her head. “Mortals get crazy ideas. They don’t know the statues are sacred to Zeus, but they know there’s something special about them.”

“When you were here last, did they talk to you or anything?”

Thalia’s expression darkened. I could tell that she’d come here before hoping for exactly that—some kind of sign from her dad. Some connection. “No. They don’t do anything. They’re just big metal statues.”

I thought about the last big metal statue we’d run into. That hadn’t gone so well. But I decided not to bring it up.

“Let us find the dam snack bar,” Zoë said. “We should eat while we can.”

Grover cracked a smile. “The dam snack bar?”

Zoë blinked. “Yes. What is funny?”

“Nothing,” Grover said, trying to keep a straight face. “I could use some dam french fries.”

Even Thalia smiled at that. “And I need to use the dam restroom.”

Maybe it was the fact that we were so tired and strung out emotionally, but I started cracking up, and Thalia and Grover joined in, while Zoë just looked at us. “I do not understand.”

“I want to use the dam water fountain,” Grover said.

“And . . .” Thalia tried to catch her breath. “I want to buy a dam T-shirt.”

I busted up, and I probably would’ve kept laughing all day, but then I heard a noise:

“Moooo.”

The smile melted off my face. I wondered if the noise was just in my head, but Grover had stopped laughing too. He was looking around, confused. “Did I just hear a cow?”

“A dam cow?” Thalia laughed.

“No,” Grover said. “I’m serious.”

Zoë listened. “I hear nothing.”

Thalia was looking at me. “Percy, are you okay?”

“Yeah,” I said. “You guys go ahead. I’ll be right in.”

“What’s wrong?” Grover asked.

“Nothing,” I said. “I . . . I just need a minute. To think.”

They hesitated, but I guess I must’ve looked upset, because they finally went into the visitor center without me. As soon as they were gone, I jogged to the north edge of the dam and looked over.

“Moo.”

She was about thirty feet below in the lake, but I could see her clearly: my friend from Long Island Sound, Bessie the cow serpent.

I looked around. There were groups of kids running along the dam. A lot of senior citizens. Some families. But nobody seemed to be paying Bessie any attention yet.

“What are you doing here?” I asked her.

“Moo!”

Her voice was urgent, like she was trying to warn me of something.

“How did you get here?” I asked. We were thousands of miles from Long Island, hundreds of miles inland. There was no way she could’ve swum all the way here. And yet, here she was.

Bessie swam in a circle and butted her head against the side of the dam. “Moo!”

She wanted me to come with her. She was telling me to hurry.

“I can’t,” I told her. “My friends are inside.”

She looked at me with her sad brown eyes. Then she gave one more urgent “Mooo!,” did a flip, and disappeared into the water.

I hesitated. Something was wrong. She was trying to tell me that. I considered jumping over the side and following her, but then I tensed. The hairs on my arms bristled. I looked down the dam road to the east and I saw two men walking slowly toward me. They wore gray camouflage outfits that flickered over skeletal bodies.

They passed through a group of kids and pushed them aside. A kid yelled, “Hey!” One of the warriors turned, his face changing momentarily into a skull.

“Ah!” the kid yelled, and his whole group backed away.

I ran for the visitor center.

I was almost to the stairs when I heard tires squeal. On the west side of the dam, a black van swerved to a stop in the middle of the road, nearly plowing into some old people.

The van doors opened and more skeleton warriors piled out. I was surrounded.

I bolted down the stairs and through the museum entrance. The security guard at the metal detector yelled, “Hey, kid!” But I didn’t stop.

I ran through the exhibits and ducked behind a tour group. I looked for my friends, but I couldn’t see them anywhere. Where was the dam snack bar?

“Stop!” The metal-detector guy yelled.

There was no place to go but into an elevator with the tour group. I ducked inside just as the door closed.

“We’ll be going down seven hundred feet,” our tour guide said cheerfully. She was a park ranger, with long black hair pulled back in a ponytail and tinted glasses. I guess she hadn’t noticed that I was being chased. “Don’t worry, ladies and gentlemen, the elevator hardly ever breaks.”

“Does this go to the snack bar?” I asked her.

A few people behind me chuckled. The tour guide looked at me. Something about her gaze made my skin tingle.

“To the turbines, young man,” the lady said. “Weren’t you listening to my fascinating presentation upstairs?”

“Oh, uh, sure. Is there another way out of the dam?”

“It’s a dead end,” a tourist behind me said. “For heaven’s sake. The only way out is the other elevator.”

The doors opened.

“Go right ahead, folks,” the tour guide told us. “Another ranger is waiting for you at the end of the corridor.”

I didn’t have much choice but to go out with the group.

“And young man,” the tour guide called. I looked back. She’d taken off her glasses. Her eyes were startlingly gray, like storm clouds. “There is always a way out for those clever enough to find it.”

The doors closed with the tour guide still inside, leaving me alone.

Before I could think too much about the woman in the elevator, a ding came from around the corner. The second elevator was opening, and I heard an unmistakable sound— the clattering of skeleton teeth.