As a test, she tied one end around a support column and leaned on the rope with all her weight. The plastic swords bent under her, but they provided some extra bulk to the knots in the cord, so at least she could keep a better grip.

The ladder wouldn’t win any design awards, but it might get her to the bottom of the cavern safely. First, she stuffed her backpack with the leftover spools of string. She wasn’t sure why, but they were one more resource, and not too heavy.

She headed back to the hole in the mosaic floor. She secured one end of her ladder to the nearest piece of scaffolding, lowered the rope into the cavern, and shinnied down.

Chapter 34

As Annabeth hung in the air, descending hand over hand with the ladder swinging wildly, she thanked Chiron for all those years of training on the climbing course at Camp Half-Blood. She’d complained loudly and often that rope climbing would never help her defeat a monster. Chiron had just smiled, like he knew this day would come.

Finally Annabeth made it to the bottom. She missed the brickwork edge and landed in the canal, but it turned out to be only a few inches deep. Freezing water soaked into her running shoes.

She held up her glowing dagger. The shallow channel ran down the middle of a brickwork tunnel. Every few yards, ceramic pipes jutted from the walls. She guessed that the pipes were drains, part of the ancient Roman plumbing system, though it was amazing to her that a tunnel like this had survived, crowded underground with all the other centuries’ worth of pipes, basements, and sewers.

A sudden thought chilled her even more than the water. A few years ago, Percy and she had gone on a quest in Daedalus’s labyrinth—a secret network of tunnels and rooms, heavily enchanted and trapped, which ran under all the cities of America.

When Daedalus died in the Battle of the Labyrinth, the entire maze had collapsed—or so Annabeth believed. But what if that was only in America? What if this was an older version of the labyrinth? Daedalus once told her that his maze had a life of its own. It was constantly growing and changing. Maybe the labyrinth could regenerate, like monsters. That would make sense. It was an archetypal force, as Chiron would say—something that could never really die.

If this was part of the labyrinth…

Annabeth decided not to dwell on that, but she also decided not to assume her directions were accurate. The labyrinth made distance meaningless. If she wasn’t careful, she could walk twenty feet in the wrong direction and end up in Poland.

Just to be safe, she tied a new ball of string to the end of her rope ladder. She could unravel it behind her as she explored. An old trick, but a good one.

She debated which way to go. The tunnel seemed the same in both directions. Then, about fifty feet to her left, the Mark of Athena blazed against the wall. Annabeth could swear it was glaring at her with those big fiery eyes, as if to say, What’s your problem? Hurry up!

She was really starting to hate that owl.

By the time she reached the spot, the image had faded, and she’d run out of string on her first spool.

As she was attaching a new line, she glanced across the tunnel. There was a broken section in the brickwork, as if a sledgehammer had knocked a hole in the wall. She crossed to take a look. Sticking her dagger through the opening for light, Annabeth could see a lower chamber, long and narrow, with a mosaic floor, painted walls, and benches running down either side. It was shaped sort of like a subway car.

She stuck her head into the hole, hoping nothing would bite it off. At the near end of the room was a bricked-off doorway. At the far end was a stone table, or maybe an altar.

Hmm…The water tunnel kept going, but Annabeth was sure this was the way. She remembered what Tiberinus had said: Find the altar of the foreign god. There didn’t seem to be any exits from the altar room, but it was a short drop onto the bench below. She should be able to climb out again with no problem.

Still holding her string, she lowered herself down.

The room’s ceiling was barrel-shaped with brick arches, but Annabeth didn’t like the look of the supports. Directly above her head, on the arch nearest to the bricked-in doorway, the capstone was cracked in half. Stress fractures ran across the ceiling. The place had probably been intact for two thousand years, but she decided she’d rather not spend too much time here. With her luck, it would collapse in the next two minutes.

The floor was a long narrow mosaic with seven pictures in a row, like a time line. At Annabeth’s feet was a raven. Next was a lion. Several others looked like Roman warriors with various weapons. The rest were too damaged or covered in dust for Annabeth to make out details. The benches on either side were littered with broken pottery. The walls were painted with scenes of a banquet: a robed man with a curved cap like an ice cream scoop, sitting next to a larger guy who radiated sunbeams. Standing around them were torchbearers and servants, and various animals like crows and lions wandered in the background. Annabeth wasn’t sure what the picture represented, but it didn’t remind her of any Greek legends that she knew.

At the far end of the room, the altar was elaborately carved with a frieze showing the man with the ice-cream-scoop hat holding a knife to the neck of a bull. On the altar stood a stone figure of a man sunk to his knees in rock, a dagger and a torch in his outraised hands. Again, Annabeth had no idea what those images meant.

She took one step toward the altar. Her foot went CRUNCH. She looked down and realized she’d just put her shoe through a human rib cage.

Annabeth swallowed back a scream. Where had that come from? She had glanced down only a moment before and hadn’t seen any bones. Now the floor was littered with them. The rib cage was obviously old. It crumbled to dust as she removed her foot. Nearby lay a corroded bronze dagger very much like her own. Either this dead person had been carrying the weapon, or it had killed him.

She held out her blade to see in front of her. A little farther down the mosaic path sprawled a more complete skeleton in the remains of an embroidered red doublet, like a man from the Renaissance. His frilled collar and skull had been badly burned, as if the guy had decided to wash his hair with a blowtorch.

Wonderful, Annabeth thought. She lifted her eyes to the altar statue, which held a dagger and a torch.

Some kind of test, Annabeth decided. These two guys had failed. Correction: not just two guys. More bones and scraps of clothing were scattered all the way to the altar. She couldn’t guess how many skeletons were represented, but she was willing to bet they were all demigods from the past, children of Athena on the same quest.

“I will not be another skeleton on your floor,” she called to the statue, hoping she sounded brave.

A girl, said a watery voice, echoing through the room. Girls are not allowed.

A female demigod, said a second voice. Inexcusable.

The chamber rumbled. Dust fell from the cracked ceiling. Annabeth bolted for the hole she’d come through, but it had disappeared. Her string had been severed. She clambered up on the bench and pounded on the wall where the hole had been, hoping the hole’s absence was just an illusion, but the wall felt solid.

She was trapped.

Along the benches, a dozen ghosts shimmered into existence—glowing purple men in Roman togas, like the Lares she’d seen at Camp Jupiter. They glared at her as if she’d interrupted their meeting.

She did the only thing she could. She stepped down from the bench and put her back to the bricked-in doorway. She tried to look confident, though the scowling purple ghosts and the demigod skeletons at her feet made her want to turtle in her T-shirt and scream.

“I’m a child of Athena,” she said, as boldly as she could manage.

“A Greek,” one of the ghosts said with disgust. “That is even worse.”

At the other end of the chamber, an old-looking ghost rose with some difficulty (do ghosts have arthritis?) and stood by the altar, his dark eyes fixed on Annabeth. Her first thought was that he looked like the pope. He had a glittering robe, a pointed hat, and a shepherd’s crook.

“This is the cavern of Mithras,” said the old ghost. “You have disturbed our sacred rituals. You cannot look upon our mysteries and live.”

“I don’t want to look upon your mysteries,” Annabeth assured him. “I’m following the Mark of Athena. Show me the exit, and I’ll be on my way.”

Her voice sounded calm, which surprised her. She had no idea how to get out of here, but she knew she had to succeed where her siblings had failed. Her path led farther on—deeper into the underground layers of Rome.

The failures of your predecessors will guide you, Tiberinus had said. After that…I do not know.

The ghosts mumbled to each other in Latin. Annabeth caught a few unkind words about female demigods and Athena.

Finally the ghost with the pope hat struck his shepherd’s crook against the floor. The other Lares fell silent.

“Your Greek goddess is powerless here,” said the pope. “Mithras is the god of Roman warriors! He is the god of the legion, the god of the empire!”

“He wasn’t even Roman,” Annabeth protested. “Wasn’t he, like, Persian or something?”

“Sacrilege!” the old man yelped, banging his staff on the floor a few more times. “Mithras protects us! I am the pater of this brotherhood—”

“The father,” Annabeth translated.

“Do not interrupt! As pater, I must protect our mysteries.”

“What mysteries?” Annabeth asked. “A dozen dead guys in togas sitting around in a cave?”

The ghosts muttered and complained, until the pater got them under control with a taxicab whistle. The old guy had a good set of lungs. “You are clearly an unbeliever. Like the others, you must die.”

The others. Annabeth made an effort not to look at the skeletons.

Her mind worked furiously, grasping for anything she knew about Mithras. He had a secret cult for warriors. He was popular in the legion. He was one of the gods who’d supplanted Athena as a war deity. Aphrodite had mentioned him during their teatime chat in Charleston. Aside from that, Annabeth had no idea. Mithras just wasn’t one of the gods they talked about at Camp Half-Blood. She doubted the ghosts would wait while she whipped out Daedalus’s laptop and did a search.

She scanned the floor mosaic—seven pictures in a row. She studied the ghosts and noticed all of them wore some sort of badge on their toga—a raven, or a torch, or a bow.

“You have rites of passage,” she blurted out. “Seven levels of membership. And the top level is the pater.”

The ghosts let out a collective gasp. Then they all began shouting at once.

“How does she know this?” one demanded.

“The girl has gleaned our secrets!”

“Silence!” the pater ordered.

“But she might know about the ordeals!” another cried.

“The ordeals!” Annabeth said. “I know about them!”

Another round of incredulous gasping.

“Ridiculous!” The pater yelled. “The girl lies! Daughter of Athena, choose your way of death. If you do not choose, the god will choose for you!”

“Fire or dagger,” Annabeth guessed.

Even the pater looked stunned. Apparently he hadn’t remembered there were victims of past punishments lying on the floor.