She smiled, and for a second, I remembered that I actually liked her when she wasn't yelling at me. She turned and helped Grover get their canoe into the water.

As it turned out, I didn't even need to control the currents. As soon as we got in the river, I looked over the edge of the boat and found a couple of naiads staring at me.

They looked like regular teenage girls, the kind you'd see in any mall, except for the fact that they were underwater.

Hey, I said.

They made a bubbling sound that may have been giggling. I wasn't sure. I had a hard time understanding naiads.

We're heading upstream, I told them. Do you think you could—

Before I could even finish, the naiads each chose a canoe and began pushing us up the river. We started so fast

Grover fell into his canoe with his hooves sticking up in the air.

"I hate naiads," Zoe grumbled.

A stream of water squirted up from the back of the boat and hit Zoe in the face.

"She-devils!" Zoe went for her bow.

"Whoa," I said. "They're just playing."

"Cursed water spirits. They've never forgiven me."

"Forgiven you for what?"

She slung her bow back over her shoulder. "It was a long time ago. Never mind."

We sped up the river, the cliffs looming up on either side of us.

"What happened to Bianca wasn't your fault," I told her. "It was my fault. I let her go."

I figured this would give Zoe an excuse to start yelling at me. At least that might shake her out of feeling depressed.

Instead, her shoulders slumped. "No, Percy. I pushed her into going on the quest. I was too anxious. She was a powerful half-blood. She had a kind heart, as well. I… I thought she would be the next lieutenant."

"But you're the lieutenant."

She gripped the strap of her quiver. She looked more tired than I'd ever seen her. "Nothing can last forever, Percy. Over two thousand years I have led the Hunt, and my wisdom has not improved. Now Artemis herself is in danger."

"Look, you can't blame yourself for that."

"If I had insisted on going with her—"

"You think you could've fought something powerful enough to kidnap Artemis? There's nothing you could have done."

Zoe didn't answer.

The cliffs along the river were getting taller. Long shadows fell across the water, making it a lot colder, even though the day was bright.

Without thinking about it, I took Riptide out of my pocket. Zoe looked at the pen, and her expression was pained.

"You made this," I said.

"Who told thee?"

"I had a dream about it."

She studied me. I was sure she was going to call me crazy, but she just sighed. "It was a gift. And a mistake."

"Who was the hero?" I asked.

Zoe shook her head. "Do not make me say his name. I swore never to speak it again."

"You act like I should know him."

"I am sure you do, hero. Don't all you boys want to be just like him?"

Her voice was so bitter, I decided not to ask what she meant. I looked down at Riptide, and for the first time, I wondered if it was cursed.

"Your mother was a water goddess?" I asked.

"Yes, Pleione. She had five daughters. My sisters and I. The Hesperides."

"Those were the girls who lived in a garden at the edge of the West. With the golden apple tree and a dragon guarding it."

"Yes," Zoe said wistfully. "Ladon."

"But weren't there only four sisters'?"

"There are now. I was exiled. Forgotten. Blotted out as if I never existed."

"Why?"

Zoe pointed to my pen. "Because I betrayed my family and helped a hero. You won't find that in the legend either. He never spoke of me. After his direct assault on Ladon failed, I gave him the idea of how to steal the apples, how to trick my father, but he took all the credit."

"But—"

Gurgle, gurgle, the naiad spoke in my mind. The canoe was slowing down.

I looked ahead, and I saw why.

This was as far as they could take us. The river was blocked. A dam the size of a football stadium stood in our path.

"Hoover Dam," Thalia said. "It's huge."

We stood at the river's edge, looking up at a curve of concrete that loomed between the cliffs. People were walking along the top of the dam. They were so tiny they looked like fleas.

The naiads had left with a lot of grumbling—not in words I could understand, but it was obvious they hated this dam blocking up their nice river. Our canoes floated back downstream, swirling in the wake from the dam's discharge vents.

"Seven hundred feet tall," I said. "Built in the 1930s."

"Five million cubic acres of water," Thalia said.

Graver sighed. "Largest construction project in the United States."

Zoe stared at us. "How do you know all that?"

"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. Zoe 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," Zoe 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," Zoe said. "We should eat while we can."

Grover cracked a smile. "The dam snack bar?"

Zoe 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 Zoe 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."

Zoe 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.