The curly-haired boy was bobbing his head up and down to whatever music was in his download file. Suddenly, he noticed that everyone was staring at him. Looking shy, he retreated to the window. Hollis wondered if Kröte was going to return to the ledge and escape.

“Does he speak English?” Hollis asked.

“Just a few words.” He turned to his cousin. “Kröte! Speak English!”

“Multidimensional,” the boy whispered.

“Sehr gut!” Tristan smiled proudly. “He learned that on the Internet.”

“And is that how you heard about the Shadow Program?”

“No. It was from the Free Runner community. Our friend Ingrid was working for a company called Personal Customer. I guess she was good at her job, because a man named Lars Reichhardt asked her to work for his division. Each person on the team was given a small job and told not to share information with their colleagues. Two weeks ago, Ingrid got access to another part of the system and found out about the Shadow Program. Then we got the e-mail from the British Free Runners.”

“Hollis and I need to get into the computer center,” Mother Blessing said. “Can you help us?”

“Of course!” Tristan extended his hands as if he were offering them a gift. “We’ll take you all the way.”

“Do we have to climb up walls?” Mother Blessing asked. “I didn’t bring any ropes.”

“Ropes are not necessary. We’re going beneath the streets. During World War Two, thousands of bombs fell on Berlin, but Hitler was safe in his bunker. Most of the bunkers and tunnels are still there. Kröte has been exploring the system since he was nine years old.”

“I guess you guys don’t have time for school,” Hollis said.

“We go to school-sometimes. The girls are there, and I like to play football.”

THE FOUR OF them left Ballhaus a few minutes later and crossed the river. Kröte was carrying a nylon backpack that contained his equipment for going underground. Looking like a wild-haired Boy Scout, he kept darting ahead of his cousin.

After walking down a wide avenue that bordered the Tiergarten, they reached the Memorial to the Murdered Jews of Europe. The Holocaust memorial was a large, sloping grid covered with concrete slabs of different heights. Hollis thought they looked like thousands of gray coffins. Tristan explained that the antigraffiti chemical painted on the slabs was provided by an affiliate of the company that had manufactured the Zyklon-B used in the death chambers.

“For war, they made poison gas. For peace, they fight taggers.” Tristan shrugged. “It’s all part of the Vast Machine.”

A row of souvenir shops and cafés was directly across the street from the memorial. The building looked like a flimsy structure created with plywood and a few pieces of glass. Kröte ran past a Dunkin’ Donuts shop and disappeared around the corner of the building. They found the boy unlocking a padlock on a steel hatch cover set flush to the concrete.

“Where’d you get the key?” Mother Blessing asked.

“We cut the city lock a year ago and put on a substitute.”

Kröte opened his knapsack and took out three flashlights. For his own use, he slipped on a headlamp with a high-intensity lightbulb.

They pulled open the hatch and hurried down a steel ladder. Hollis climbed with one hand on the rungs while he held the equipment bag to his chest. They reached a maintenance tunnel filled with communications cables, and Kröte unfastened another padlock on an unmarked steel door.

“Why hasn’t anybody noticed that you changed the locks?” Hollis asked.

“Nobody official wants to enter this place-just explorers like us. It’s dark and scary down here. It’s altes Deutschland. The past.”

One by one, they passed through the doorway to a corridor with a concrete floor. Now they were directly below the memorial, standing in the bunker used by Joseph Goebbels and his staff during the bombing raids. Hollis had been expecting something a bit more impressive-dust-covered office furniture and a Nazi banner hanging on the wall. Instead, their little pool of light illuminated concrete-block walls coated with a grayish-white paint and the words Rauchen Verboten. No smoking.

“The paint is fluorescent. After all these years, it still works.”

Kröte paced slowly down the corridor with his light beam focused on the wall. “Licht,” he said in a faint voice.

Tristan told Hollis and Mother Blessing to turn off their flashlights. In the dark they saw that Kröte’s movements had created a bright green line on the wall that glowed for three or four seconds before fading.

They switched on the flashlights again and continued through the bunker. In one room there was an old bed frame, stripped of its mattress. Another room looked like a small clinic, with a white examination table and an empty glass cabinet.

“The Russians raped the women of Berlin and looted almost everything,” Tristan said. “They stayed away from only one place in this bunker. Maybe they were too lazy or it was too horrible to see.”

“What are you talking about?” Mother Blessing asked.

“Thousands of Germans killed themselves when the Russians arrived. And where did they do it? In the toilet. It was one of the few places where you could be alone.”

Kröte was standing beside an open doorway with the word Waschraum painted on the wall. Arrows pointed in two directions: Männer and Frauen. “The bones are still in the toilet stalls,” Tristan announced. “You can see them-if you’re not frightened.”

Mother Blessing shook her head. “A waste of time.”

But Hollis was compelled to follow the boy up three steps and through a door that led to the women’s washroom. The two light beams revealed a row of wooden toilet cabinets. Their doors were closed, and Hollis felt as if they concealed the remains of more than one suicide. Kröte took a few steps forward and pointed. Near the end of the room one of the wooden doors was slightly open. A mummified hand, looking like a black claw, pushed through the gap. Hollis felt as if he had been guided into the land of the dead. His entire body shivered and he hurried back to the main corridor.

“Did you see the hand?”

“Yeah. I saw it.”

“And all Berlin is built on top of this,” Tristan said. “Built on the dead.”

“I don’t give a damn,” Mother Blessing snapped. “Let’s go.”

At the end of the corridor was another steel hatch, but this one was unlocked. Tristan grabbed the handle and pulled it open. “Now we enter the old sewage system. Because this area was near the wall, both East and West Germany left it alone.”

They climbed beneath the bunker into a drainage pipe about eight feet in diameter. Water trickled along the floor of the pipe. Their flashlights touched the surface and made it gleam. Salt stalactites came down from the top of the pipe like pieces of white string. There were white mushrooms and a strange-looking fungus that resembled yellowish globs of fat. Splashing through the water, Kröte guided them forward. When he reached a juncture and turned to wait, the light jiggled like a firefly.

Eventually they reached a much smaller pipe that emptied into the larger system. Kröte began chattering in German to his cousin, pointing at the pipe and gesturing with his hands.

“This is it. Crawl about ten meters down the drain and force your way in.”

“What are you talking about?” Mother Blessing glared at Tristan. “You promised to take us all the way.”

“We’re not going into a Tabula computer center,” Tristan said. “It’s too dangerous.”

“The real danger is in front of you, young man. I dislike people who don’t deliver what they promise…”

“But we’re doing you a favor!”

“That’s your interpretation, not mine. All I know is that you accepted an obligation.”

The coldness in the Harlequin’s eyes and the precise way she spoke were intimidating. Tristan stopped dancing around, frozen in the middle of the tunnel. Kröte glanced at his cousin and looked frightened.