"Whoa," he said. "The Lucian crest."

Nellie squinted. "That? That looks like one of those ladies on a trucker's mud flaps."

"No, it's two snakes around a sword," Amy said. "Trust me, if you'd seen the Lucian crest, you'd recognize it."

"There's more," Alistair said. "Look at the paper Franklin is holding. Turn it upside down. There - brushed over with white paint, almost impossible to read."

Dan never would've noticed if Alistair hadn't said something, but when he looked closely, he could see the faint shadow of words on the document in Franklin's hand.

"'Paris,'" he read. "'1785.'"

"Exactly, my dear boy: a painting of Franklin with a key, the Cahill family crest, and the words Paris, 1785. A significant hint."

"I never would've found this," Amy said in amazement.

Alistair shrugged. "As you said, my dear, there are many possible hints, all leading us along the path to the second clue. Unfortunately, we Cahills would rather fight each other, steal information, and keep each other from getting ahead" -- he shifted his weight and winced -- "as my cracked rib and black eyes will testify."

"But who buried all these hints in the first place?" Amy demanded. "Franklin?"

Alistair sipped his espresso. "I don't know, my dear. If I were to guess, I'd say it is a hodgepodge, a collected effort by many Cahills over the centuries. Dear old Grace seems to be the one who wove them all together, though why or how, I don't know.

Whatever the final treasure is, the greatest Cahill minds have gone to a good deal of trouble to hide it. Or perhaps, as in the case of Benjamin Franklin, some of them are trying to lead us toward it. I suppose we will only know for certain when we find the treasure."

"We?"

Dan said.

"I still believe we must have an alliance," Alistair said.

"Uh-uh." Nellie shook her head. "Don't trust this guy, kiddos. He's too smooth."

Alistair laughed. "And you're an expert on smooth, my teenage babysitter?"

"Au pair!" Nellie corrected.

Alistair looked like he wanted to make another joke at her expense. Then he glanced at her lethal backpack and apparently changed his mind.

"The point is, children, our competitors have decided you are the team to beat."

"But why us?" Amy demanded.

Alistair shrugged. "You've been ahead of the game so far. You have escaped every trap. You were always Grace's favorites." His eyes glittered, like a starving man looking at a Big Mac. "Let's be honest, eh? We all believe Grace gave you inside information.

She must have. Tell me what it is, and I can help you."

Dan clenched his fists. He remembered that video of Grace, how stunned he'd felt when she'd announced the contest. Grace should have given them inside information. If she'd really loved them, she wouldn't have left them in the dark. The other teams were after them now because they thought Amy and Dan were Grace's favorites. But apparently Grace hadn't cared about them. They were just another team in this big cruel game she'd cooked up. The more he thought about it, the more betrayed he felt. He looked at the jade necklace around Amy's neck.

He wanted to yank it off and throw it away. His eyes started to burn.

"We don't have inside information," he mumbled.

"Come now, my boy," Alistair said. "You are in danger. I could protect you. We could search the Catacombs together."

"We'll search by ourselves," Dan said.

"As you wish, my boy. But be aware: The Catacombs are huge. There are miles of tunnels. Most aren't even mapped. You can easily get lost down there. Special police patrol it to keep out trespassers. Some of the tunnels are flooded. Others collapse from time to time. Searching for Franklin's clue in the Catacombs will be dangerous and futile unless" -- he leaned forward and raised his eyebrows -- "unless you do know something you haven't told me. The almanac had a note in the margin. It mentioned coordinates in a box. You wouldn't happen to know what this box might be?"

"Even if we knew," Dan said, "we wouldn't tell you."

Amy touched the jade necklace at her collar. "Sorry, Uncle Alistair."

"I see." Alistair sat back. "I admire your spirit. But what if I were to ... trade information? I'm sure you are wondering about those notes your mother made. I knew your parents. I could explain a few things."

Dan felt as if the air had turned to glass. He was afraid to move or he might get cut.

"What few things?"

Alistair smiled, like he knew he'd hooked them. "Your mother's interest in the clues, perhaps. Or what your father really did for a living."

"He was a math professor," Amy said.

"Mmm." Alistair's smile was so irritating Dan was tempted to tell Nellie to whack him with the backpack again. "Maybe you'd like to know about the night they died?"

The turkey-and-cheese sandwich churned in Dan's stomach. "What do you know about that?"

"Many years ago, your mother -- " Alistair stopped abruptly. His eyes fixed on something across the street. "Children, we must continue this later. I believe you should look in the Catacombs by yourselves. I'll stay behind, as a show of good faith."

"What do you mean?" Dan demanded.

Alistair pointed with his cane. A hundred yards down the street, Ian and Natalie Kabra were pushing through the crowd, hurrying toward the Catacombs entrance.

"I'll hold them off as long as I can," Alistair promised. "Now get underground quickly!"

CHAPTER 16

Amy hated crowds, but the idea of plunging into the middle of seven million dead people didn't bother her.

Nellie, Dan, and she hurried down a metal staircase. They found themselves in a limestone corridor with metal pipes running overhead and dim electric lights. The warm air smelled of mildew and wet rock.

"Only one exit, guys," Nellie said nervously. "If we get caught down here -- "

"The tunnel should branch out soon," Amy said, trying to sound more confident than she felt.

The stone walls were etched with graffiti. Some looked recent, some ancient. One inscription was engraved on a marble slab right above their heads.

"Stop, mortals," Nellie translated.

"This is the empire of death."

"Cheerful," Dan muttered.

They kept walking. The floor under Amy's feet was slushy gravel. Amy was still thinking about Uncle Alistair. Had he really known something about their parents, or was he just manipulating them? She tried to put it out of her mind.

"Where are the bones?" Dan asked. Then they turned a corner into a large room and Dan said, "Oh."

It was the creepiest place Amy had ever seen. Against the walls, human bones were stacked like firewood from the floor to above Amy's head. The remains were yellow and brown -- mostly leg bones, but skulls stared out here and there like patches on a quilt. A line of skulls topped each stack.

Amy walked in awed silence. The next room was the same as the first -- wall after wall of moldering remains. Dim electric lights cast eerie shadows over the dead, making their empty eye sockets look even scarier.

"Gross," Nellie managed. "There's, like, thousands."

"Millions," Amy said. "This is only one small part."

"They dug all these people up?" Dan asked. "Who would want that job?"

Amy didn't know, but she was amazed how the workers had made patterns with skulls in the stacks of femurs -- diagonals, stripes, connect-the-dot shapes. In a weird, horrible way, it was almost beautiful.

In the third room, they found a stone altar with unlit candles.

"We need to find the oldest section," Amy said. "These bones are too recent. Look at the plaque. It's from 1804."

She led the way. Eyeless sockets of the dead seemed to stare at them as they passed.

"These are cool," Dan decided. "Maybe I could -- "

"No, Dan," Amy said. "You can't collect human bones."

"Awww."

Nellie mumbled something that sounded like a prayer in Spanish. "Why would Benjamin Franklin want to come down here?"

"He was a scientist." Amy kept walking, reading the dates on the brass plaques. "He liked public works projects. This would've fascinated him."

"Millions of dead people," Nellie said. "Real fascinating."

They turned down a narrow corridor and found themselves facing a metal gate. Amy shook the bars. The gate creaked open like it hadn't been used in hundreds of years.

"Are you sure we should go down there?" Nellie asked.

Amy nodded. The dates were getting older. On the other hand, there were no metal pipes on the ceiling up ahead, which meant no electric lights.

"Anybody got a flashlight?" she asked.

"Yeah," Nellie said. "On my keychain."

She pulled out her keys and handed them to Amy. There was a little push-button pin light. Not much, but better than nothing. They kept going. After a hundred feet, they emerged in a small room with only one other exit.

Amy shone the flashlight on an old plaque framed in skulls. "1785! These have to be the first bones put down here."

The wall they were looking at was in bad shape. The bones were brown and crumbly, and some had scattered across the floor. The skulls along the top had been crushed, though the ones quilted into the wall looked fairly intact. They were done in a square pattern -- nothing exciting.

"Search around," Amy said. "It has to be here."

Dan stuck his hands into some of the gaps in the bone wall. Nellie checked the top of the stack. Amy looked into the skulls' eye sockets with the flashlight, but she saw nothing.

"It's hopeless," she said at last. "If there was anything here, another team must've found it."

Dan scratched his head. Then he scratched a skull's head. "Why are they numbered?"

Amy wasn't in the mood for his games. "What numbers?"

"Here on the forehead." Dan tapped one of the skulls. "This guy was number three.

Were they on a football team or something?"

Amy leaned in closer. Dan was right. The number was very faint, but scratched into the skull's forehead, like someone had carved it with a knife, was the Roman numeral III.

She examined the skull below it. XIX. A square pattern. Skulls with numbers. "Check them all. Quick!"

It didn't take long. There were sixteen skulls woven into the pile of bones, done in four rows and four columns. Three of the skulls didn't have numbers. The rest did. They looked like this:

[proofreader's note: the numbers on the skulls are

XVIII

III

VI

IV

XIX

XI

I

XV

IX

XVI

II

VIII

XIII

A few of the skulls have no numbers on them.]

A chill went down Amy's back. "Coordinates in a box. A magic box!"

"What?" Dan said. "What magic?"

"Dan, can you memorize these numbers and their placement?"

"I already have."

"We need to get out of here and find a map. This is the clue -- well, the clue to the real clue, whatever Franklin was hiding."