"Wait a sec," Nellie said. "Franklin scratched numbers on skulls. Why?"

"It's a magic box," Amy said. "Franklin used to play with numbers when he got bored.

Like when he was sitting in the Philadelphia Assembly and he didn't want to listen to the dull speeches, he would create magic boxes, number problems for himself. He would fill in the missing numbers. The sums had to match, horizontally and vertically."

Nellie scowled. "You're telling me Benjamin Franklin invented sudoku?"

"Well, yeah, in a way. And these -- "

"Are coordinates," Dan supplied. "The missing sums show the location of the next clue."

Clapping echoed through the room. "Bravo."

Amy turned. Standing in the entrance were Ian and Natalie Kabra.

"I told you they could do it," Ian said to his sister.

"Oh, I suppose," Natalie conceded. Amy hated that even underground in a room full of bones, Natalie managed to look glamorous. She was wearing a black velour catsuit, so she looked eleven going on twenty-three. Her hair hung loose around her shoulders. The only part of her outfit that didn't match was the tiny silver dart gun in her hand. "Perhaps it wasn't all bad that Irina failed us."

"You!"

Dan yelled. "You convinced Irina to set us up at the ?le St-Louis. You almost got us buried in cement!"

"A shame it didn't work," Natalie said. "You would've made a fine welcome mat for the mausoleum."

"But -- but why?" Amy stammered.

Ian smiled. "To put you out of commission, of course. And to give us extra time to find this place. We needed to make sure this wasn't some clever misdirection by our dear cousin Irina. I should've noticed the magic box earlier. Thanks for your help, Amy.

Now, if you'll move aside, we'll just copy down those numbers and be off."

Amy took a shaky breath. "No."

Ian laughed. "Isn't she cute, Natalie? Acting like she has a choice."

"Yes." Natalie wrinkled her nose. "Cute."

Amy blushed. The Kabras always made her feel so awkward and stupid, but she couldn't let them get the clue. She snatched up a leg bone. "One move and I'll -- I'll crush the skulls. You'll never get the numbers."

It didn't sound like a very convincing threat, even to her, but Ian paled. "Now let's not be stupid, Amy. I know how nervous you get, but we won't hurt you."

"Not at all," Natalie agreed. She pointed her dart gun at Amy's face. "I think poison six will be adequate. Nothing lethal. Just a deep, deep sleep. I'm sure someone will find you down here ... someday."

A shadow loomed up behind the Kabras. Suddenly, Uncle Alistair charged into the room and knocked

Natalie to the ground. Her dart gun skittered away and Ian dove after it. "Run!" Alistair yelled.

Amy didn't argue. She, Nellie, and Dan raced through the other exit, into the darkness -- deeper into the Catacombs.

They ran for what seemed like hours, with nothing but the pin light to guide them.

They turned down one corridor and found it blocked by a mound of rubble. They doubled back and followed another tunnel until it submerged completely in murky yellow water. Soon, Amy had no idea which direction they were heading.

"Alistair said there are police down here," she murmured. "I wish one would find us."

But they saw no one. The little flashlight started to dim.

"No," Amy said. "No, no, no!"

They forged ahead. Fifty feet, sixty feet, and their light went out completely.

Amy found Dan's hand and squeezed it tight.

"It's going to be fine, kiddos," Nellie said, but her voice was quavering. "We can't be lost down here forever."

Amy didn't see why not. The Catacombs went on for miles and had never been mapped completely. There was no reason anyone would come looking for them.

"We could shout for help," Dan said.

"It won't do any good," Amy said gloomily. "I'm sorry, guys. This is not how I wanted things to end."

"It's not the end!" Dan said. "We could follow one wall until we find another exit. We could -- "

"Shhh!" Amy said.

"I'm just saying -- "

"Dan, seriously! Be quiet! I thought I heard something."

The tunnel was silent except for the distant drip of water. Then Amy heard it again -- a faint rumbling from somewhere in front of them.

"A train?" Nellie asked.

Amy's spirits lifted. "We must be near a Métro station. Come on!"

She shuffled forward with her hands outstretched. She shuddered as she touched a wall of bones, but she followed the corridor as it twisted to the right. Gradually, the rumbling sounds grew louder. Amy groped to the left. Her hand touched metal.

"A door!" she cried. "Dan, there's some kind of lock mechanism here. Come here and figure this out."

"Where?"

She found him in the dark and guided his hands to the lock. Within seconds, the hatch creaked open and they were blinded by electric light.

It took Amy a few moments to comprehend what she was seeing. The hatch was more like a window than a door -- a square opening about five feet off the ground, just big enough to crawl through if they climbed up to it. They were eye level with the side of some railroad tracks -- metal rails on wooden ties. And something brown and furry was scampering over the gravel bed.

Amy jumped. "A rat!"

The rodent regarded her, clearly unimpressed, then scurried on its way.

"It's a rail pit," Dan said. "We can climb out and-"

The light got brighter. The whole tunnel rumbled. Amy fell back and cupped her ears against the sound -- like a herd of dinosaurs. A train blasted past in a blur of metal wheels. It sucked the air right out of their tunnel, pulling her clothes and hair toward the hatch. Then, just as suddenly, it was gone.

When she was sure her voice would work again, she said, "We can't go out there! We'll get killed!"

"Look," Dan said. "There's a service ladder about five feet down. We'll crawl up to the rails, run to the ladder, and climb to the platform. Easy!"

"That's not easy! What if another train comes?"

"We can time it," Nellie suggested. "I've got a clock on my iPod...."

She pulled it out of her pocket, but she'd hardly pressed the wheel before another train roared by.

Nellie's glittery eye shadow made her face look ghostly in the dim light. "That was less than five minutes. The rails must be for express trains. We'll have to hurry."

"Right!" Dan said, and just like that he scrambled up and out of the hatch. "Dan!" Amy shouted.

He turned, crouching on the tracks. "Come on!"

In a daze, Amy let Nellie give her a boost. With Dan's help, she crawled out. "Now help me with Nellie!" Dan said. "But watch the third rail."

Amy stiffened. Two feet away was the black electric rail that ran the trains. She knew enough about subways to understand that one touch would be worse than a thousand Franklin batteries. She helped Nellie out of the hatch, but it was a tight squeeze. They lost time. The rails hissed and clicked beneath them.

"I'm okay!" Nellie said, brushing off her clothes. "Let's get to the ladder."

Dan started to follow, but he lurched when he tried to stand, like he was caught on something.

"Dan?" Amy said.

"It's my backpack," he said. "It's wedged ..."

He tugged at it helplessly. Somehow, one strap had gotten looped around a metal rail, and the rail had shifted, clamping the pack into place.

"Leave it!" Amy cried.

Nellie was already at the ladder, yelling at them to hurry. Passengers on the platform were starting to notice them, too. They were yelling in alarm, shouting in French.

Dan slipped the backpack off his shoulder, but it was still stuck to the rail. He tugged at it and tried to open it, but he wasn't having any luck. "Now!" Nellie yelled.

Amy could feel a faint rumbling in the tracks at her feet.

"Dan!" she pleaded. "It's not important!"

"I can get it. Just another second."

"Dan, no. It's just a backpack!"

"It won't open!"

The far end of the tunnel lit up. Nellie was right above them on the platform, reaching out her hand. A lot of other passengers were doing the same, imploring them to grab hold.

"Amy!" Nellie cried. "You first!"

She didn't want to, but maybe if she went first, Dan would see reason. She grabbed Nellie's hand and Nellie hauled her from the rail pit. Immediately, Amy turned and stuck her hand out to Dan.

"Dan, please!" she called. "Now!"

The train's headlight flashed into sight. Wind rushed through the tunnel. The ground trembled.

Dan gave the backpack another tug, but it wouldn't budge. He looked at the train, and Amy saw he was crying. She didn't understand why.

"Dan, take-MY-HAND!"

She leaned out as far as she could. The train barreled down on them. With a cry of anguish, Dan grabbed her hand, and with more strength than Amy knew she had, she yanked him out of the pit so hard they tumbled over each other.

The train rushed on. When the noise died, the passengers on the platform all broke loose at once -- scolding them in French while Nellie tried to explain and apologize.

Amy didn't care what they were saying. She held her brother, who was crying harder than he had since he was little.

She looked over the edge of the pit, but the backpack was gone, swept away into the tunnels by the force of the train. They sat for a long time while Dan shivered and wiped his eyes. Eventually, the passengers lost interest in them. They drifted away or stepped onto other trains and disappeared. No police came. Pretty soon it was just Nellie, Amy, and Dan, sitting in a corner of the platform like a homeless family.

"Dan," Amy said gently. "What was in there? What did you have in the backpack?"

He sniffled and rubbed his nose. "Nothing."

It was the worst lie Amy had ever heard. Usually, she could tell what he was thinking just by looking at his face, but he was hiding his thoughts from her. She could only tell that he was miserable.

"Forget it," he said. "We don't have time."

"Are you sure -- "

"I said forget it! We need to figure out that number box before the Kabras, don't we?"

She didn't like it, but he was right. Besides, something told her that if they stayed here much longer, the police would come and start asking questions. She took one last look at the rail pit where Dan had almost died and the dark hatch that led into the Catacombs. Fear still coursed through her body, but they'd been through too much to give up now.

"Let's go, then," she said. "We've got a clue to find."

Outside it had started to rain.

By the time they found a café, Dan seemed back to normal -- or at least they'd come to a silent agreement that they would act like everything was normal. They sat under the awning to dry off while Nellie ordered food. Amy didn't think she could eat, but she was hungrier than she'd realized.

It was five in the afternoon. They'd been in the Catacombs a long time.

She shuddered as she thought about Ian and Natalie and the poison dart gun. She hoped Uncle Alistair was all right. She still didn't trust him, but there was no denying he'd saved them in the Catacombs. She had terrible thoughts of the old man lying alone and unconscious in the maze.