Getting in was easy too. Jason’s gold sword cut through the padlock, and the metal gate creaked open. No mortals saw them. No alarms went off. Stone steps spiraled down into the gloom.

“I’ll go first,” Jason said.

“No!” Piper yelped.

Both boys turned toward her.

“Pipes, what is it?” Jason asked. “That image in the blade…you’ve seen it before, haven’t you?”

She nodded, her eyes stinging. “I didn’t know how to tell you. I saw the room down there filling with water. I saw the three of us drowning.”

Jason and Percy both frowned.

“I can’t drown,” Percy said, though he sounded like he was asking a question.

“Maybe the future has changed,” Jason speculated. “In the image you showed us just now, there wasn’t any water.”

Piper wished he was right, but she suspected they wouldn’t be so lucky.

“Look,” Percy said. “I’ll check it out first. It’s fine. Be right back.”

Before Piper could object, he disappeared down the stairwell.

She counted silently as they waited for him to come back. Somewhere around thirty-five, she heard his footsteps, and he appeared at the top, looking more baffled than relieved.

“Good news: no water,” he said. “Bad news: I don’t see any exits down there. And, uh, weird news: well, you should see this.…”

They descended cautiously. Percy took the lead, with Riptide drawn. Piper followed, and Jason walked behind her, guarding their backs. The stairwell was a cramped corkscrew of masonry, no more than six feet in diameter. Even though Percy had given the “all clear,” Piper kept her eyes open for traps. With every turn of the stairs, she anticipated an ambush. She had no weapon, just the cornucopia on a leather cord over her shoulder. If worse came to worst, the boys’ swords wouldn’t do much good in such close quarters. Maybe Piper could shoot their enemies with high-velocity smoked hams.

As they wound their way underground, Piper saw old graffiti gouged into the stones: Roman numerals, names and phrases in Italian. That meant other people had been down here more recently than the Roman Empire, but Piper wasn’t reassured. If monsters were below, they’d ignore mortals, waiting for some nice juicy demigods to come along.

Finally, they reached the bottom.

Percy turned. “Watch this last step.”

He jumped to the floor of the cylindrical room, which was five feet lower than the stairwell. Why would someone design a set of stairs like that? Piper had no idea. Maybe the room and the stairwell had been built during different time periods.

She wanted to turn and exit, but she couldn’t do that with Jason behind her, and she couldn’t just leave Percy down there. She clambered down, and Jason followed.

The room was just like she’d seen it in Katoptris’s blade, except there was no water. The curved walls had once been painted with frescoes, which were now faded to eggshell white with only flecks of color. The domed ceiling was about fifty feet above.

Around the back side of the room, opposite the stairwell, nine alcoves were carved into the wall. Each niche was about five feet off the floor and big enough for a human-sized statue, but each was empty.

The air felt cold and dry. As Percy had said, there were no other exits.

“All right.” Percy raised his eyebrows. “Here’s the weird part. Watch.”

He stepped to the middle of the room.

Instantly, green and blue light rippled across the walls. Piper heard the sound of a fountain, but there was no water. There didn’t seem to be any source of light except for Percy’s and Jason’s blades.

“Do you smell the ocean?” Percy asked.

Piper hadn’t noticed at first. She was standing next to Percy, and he always smelled like the sea. But he was right. The scent of salt water and storm was getting stronger, like a summer hurricane approaching.

“An illusion?” she asked. All of a sudden, she felt strangely thirsty.

“I don’t know,” Percy said. “I feel like there should be water here—lots of water. But there isn’t any. I’ve never been in a place like this.”

Jason moved to the row of niches. He touched the bottom shelf of the nearest one, which was just at his eye level. “This stone…it’s embedded with seashells. This is a nymphaeum.”

Piper’s mouth was definitely getting drier. “A what?”

“We have one at Camp Jupiter,” Jason said, “on Temple Hill. It’s a shrine to the nymphs.”

Piper ran her hand along the bottom of another niche. Jason was right. The alcove was studded with cowries, conches, and scallops. The seashells seemed to dance in the watery light. They were ice-cold to the touch.

Piper had always thought of nymphs as friendly spirits—silly and flirtatious, generally harmless. They got along well with the children of Aphrodite. They loved to share gossip and beauty tips. This place, though, didn’t feel like the canoe lake back at Camp Half-Blood, or the streams in the woods where Piper normally met nymphs. This place felt unnatural, hostile, and very dry.

Jason stepped back and examined the row of alcoves. “Shrines like this were all over the place in Ancient Rome. Rich people had them outside their villas to honor nymphs, to make sure the local water was always fresh. Some shrines were built around natural springs, but most were man-made.”

“So…no actual nymphs lived here?” Piper asked hopefully.

“Not sure,” Jason said. “This place where we’re standing would have been a pool with a fountain. A lot of times, if the nymphaeum belonged to a demigod, he or she would invite nymphs to live there. If the spirits took up residence, that was considered good luck.”

“For the owner,” Percy guessed. “But it would also bind the nymphs to the new water source, which would be great if the fountain was in a nice sunny park with fresh water pumped in through the aqueducts—”

“But this place has been underground for centuries,” Piper guessed. “Dry and buried. What would happen to the nymphs?”

The sound of water changed to a chorus of hissing, like ghostly snakes. The rippling light shifted from sea blue and green to purple and sickly lime. Above them, the nine niches glowed. They were no longer empty.

Standing in each was a withered old woman, so dried up and brittle they reminded Piper of mummies—except mummies didn’t normally move. Their eyes were dark purple, as if the clear blue water of their life source had condensed and thickened inside them. Their fine silk dresses were now tattered and faded. Their hair had once been piled in curls, arranged with jewels in the style of Roman noblewomen, but now their locks were disheveled and dry as straw. If water cannibals actually existed, Piper thought, this is what they looked like.

“What would happen to the nymphs?” said the creature in the center niche.

She was in even worse shape than the others. Her back was hunched like the handle of a pitcher. Her skeletal hands had only the thinnest papery layer of skin. On her head, a battered wreath of golden laurels glinted in her roadkill hair.

She fixed her purple eyes on Piper. “What an interesting question, my dear. Perhaps the nymphs would still be here, suffering, waiting for revenge.”

The next time that she got a chance, Piper swore she would melt down Katoptris and sell it for scrap metal. The stupid knife never showed her the whole story. Sure, she’d seen herself drowning. But if she’d realized that nine desiccated zombie nymphs would be waiting for her, she never would’ve come down here.

She considered bolting for the stairs, but when she turned, the doorway had disappeared. Naturally. Nothing was there now but a blank wall. Piper suspected it wasn’t just an illusion. Besides, she would never make it to the opposite side of the room before the zombie nymphs could jump on them.

Jason and Percy stood to either side of her, their swords ready. Piper was glad to have them close, but she suspected their weapons wouldn’t do any good. She’d seen what would happen in this room. Somehow, these things were going to defeat them.

“Who are you?” Percy demanded.

The central nymph turned her head. “Ah…names. We once had names. I was Hagno, the first of the nine!”

Piper thought it was a cruel joke that a hag like her would be named Hagno, but she decided not to say that.

“The nine,” Jason repeated. “The nymphs of this shrine. There were always nine niches.”

“Of course.” Hagno bared her teeth in a vicious smile. “But we are the original nine, Jason Grace, the ones who attended the birth of your father.”

Jason’s sword dipped. “You mean Jupiter? You were there when he was born?”

“Zeus, we called him then,” Hagno said. “Such a squealing whelp. We attended Rhea in her labor. When the baby arrived, we hid him so that his father, Kronos, would not eat him. Ah, he had lungs, that baby! It was all we could do to drown out the noise so Kronos could not find him. When Zeus grew up, we were promised eternal honors. But that was in the old country, in Greece.”

The other nymphs wailed and clawed at their niches. They seemed to be trapped in them, Piper realized, as if their feet were glued to the stone along with the decorative seashells.

“When Rome rose to power, we were invited here,” Hagno said. “A son of Jupiter tempted us with favors. A new home, he promised. Bigger and better! No down payment, an excellent neighborhood. Rome will last forever.”

“Forever,” the others hissed.

“We gave in to temptation,” Hagno said. “We left our simple wells and springs on Mount Lycaeus and moved here. For centuries, our lives were wonderful! Parties, sacrifices in our honor, new dresses and jewelry every week. All the demigods of Rome flirted with us and honored us.”

The nymphs wailed and sighed.

“But Rome did not last,” Hagno snarled. “The aqueducts were diverted. Our master’s villa was abandoned and torn down. We were forgotten, buried under the earth, but we could not leave. Our life sources were bound to this place. Our old master never saw fit to release us. For centuries, we have withered here in the darkness, thirsty…so thirsty.”

The others clawed at their mouths.

Piper felt her own throat closing up.

“I’m sorry for you,” she said, trying to use charmspeak. “That must have been terrible. But we are not your enemies. If we can help you—”

“Oh, such a sweet voice!” Hagno cried. “Such beautiful features. I was once young like you. My voice was as soothing as a mountain stream. But do you know what happens to a nymph’s mind when she is trapped in the dark, with nothing to feed on but hatred, nothing to drink but thoughts of violence? Yes, my dear. You can help us.”

Percy raised his hand. “Uh…I’m the son of Poseidon. Maybe I can summon a new water source.”

“Ha!” Hagno cried, and the other eight echoed, “Ha! Ha!”

“Indeed, son of Poseidon,” Hagno said. “I know your father well. Ephialtes and Otis promised you would come.”

Piper put her hand on Jason’s arm for balance.