"Wow, honey," Nell suddenly realized. "Look at all the underwater species he's cataloged here."

A fuchsia and orange sphere of six-inch tongues rolled over Hell's Window above and Sasha shouted. "Hey, you guys! Here comes the sushi wagon!"

They both looked up. "Sushi wagon?" Geoffrey asked.

"It's a sushi bar on wheels! Everything loves the sushi wagon," she said. "I was wondering when one would show up. Watch!"

The buoy-sized ball rolled down the window like the sticky toys children throw at walls; and when it reached the bottom of the window, gammarids and even aggregators leaped out of nowhere to tear off the sashimi-like tongues of flesh covering the globe's surface.

"It's like a giant Volvox," said Geoffrey, shifting his bound foot that continued to throb with pain. The large ball rolled along the window's ledge with its vividly hued tongues. "Maybe it's a colony of creatures that fuse together into these spheres." Animals were attacking it from all directions.

"Everything's ripping off pieces of it. How does it survive?" Nell wondered.

"Don't worry," Sasha said. "That's how the sushi wagon makes babies. Dimitri told me that one out of a hundred pieces of sushi has eggs inside that hatch in the stomachs of the animals that eat them. They turn into new sushi balls and burst out!"

"Wow," Geoffrey said. "Now that's bad sushi."

Nell pointed at a milky slug or flatworm that was the size of a throw rug, which glided over the top edge of the window. She rose and examined its ventral surface as it shimmied over the glass. "I think I've seen one of those before."

Geoffrey noticed there were ten S-ing rows of suction cups extending around the giant flatworm's head. "I've definitely seen one of those before," he said, and he hobbled to his feet and stood closer to the window. "It's some kind of land octopus...."

"It's a ghost!" Sasha cried, clutching Nell's arm.

"No," said Nell.

"Look," Geoffrey countered, pointing at the suction cups.

"Amphipods and mollusks," Nell said.

"Both ancient groups of animals," Geoffrey agreed.

"It makes sense," Nell said. "They must have been isolated for hundreds of millions of years to diverge this radically."

"But how could this place have existed so long?" Geoffrey shook his head. "That's what puzzles me."

"Henders Island existed longer," Nell reminded him. "Back to the pre-Cambrian. One tiny fragment that made it through."

"Dimitri said the Urals are the oldest mountain range on Earth," Geoffrey recalled as the opalescent creature rippled rows of suction cups like a kaleidoscopic caterpillar moving down the glass. The creature turned, moving parallel with the bottom of the window, and about three feet from the edge, it stopped. Peeling its lower edge from the glass, the ghostly mollusk lifted the right side of its body outward.

"What is it doing?" Nell breathed.

They watched anxiously.

One of the gammies flitted past on the window ledge beneath it and, with shocking quickness, the creature slapped down and pressed the kicking amphipod against the window.

"Whoa!" Nell said.

What happened next was like a diabolical miracle: the amorphous mollusk stretched down over each jerking leg of the pinned gammarid. Suction cups on its underside clamped into each joint of the gammy's legs with some kind of beaks.

"They have suckers like colossal squids!" Geoffrey exclaimed.

"They're more like jaws," Nell whispered.

After each joint of the animal was vised by the suction clamps, the terrestrial octopus moved its head into position and crunched the amphipod's neck with knifelike blades that sawed through its nerve cord. All at once, the gammy's legs went limp.

"Dear God," Geoffrey muttered.

The paralyzed creature's legs suddenly pointed forward and then backwards across the window, flexing in unison as the mollusk seemed to be testing its control of the animal's body like a puppeteer.

"What?" Nell gasped, looking at Geoffrey.

The octopus rolled off the glass, now in full possession of its prey, and grabbed hold of the window ledge with the gammarid's long legs. The flesh of the octopus changed color before their eyes, matching the amphipod's checkered yellow-orange-and-white pattern.

"Did that just happen?" Nell asked.

Geoffrey nodded. "It's like some sort of mimic octopus," he said.

"What's a mimic octopus?" Sasha asked.

"The mimic octopus," he explained, "can fake the shape, color, and motions of more than a dozen creatures. It can make itself look and even move like a lionfish, a flounder, a sea snake, a mantis shrimp, and even brittle stars-animals from completely different branches of life. This animal might be some kind of cousin or crazy uncle of the mimic octopus."

Nell felt a deep, primal fear as she watched the ghoulish animal move the carcass of the gammarid, testing its control. "That thing attached itself to the gammy like an external muscular system!"

Geoffrey nodded. He stared at the creature, remembering what one of them had done to the guard outside the power plant. He didn't want to tell them what he had seen, and he didn't want to scare Sasha. "Octopuses are incredibly smart," he said.

"An octopus predicted the winner of the World Cup!" Sasha said.

"That's right." Geoffrey laughed.

"I hate them!" Sasha said, wrinkling her nose.

"There's a species of fungus that turns ants into zombies," Nell said as she stared at the ghost octopus moving the gammy's limbs in jerky motions now, mimicking the other gammies. "The fungus actually makes the ants cut leaves for the fungus to feed on, all through a strange kind of mind control."

"Really?" Geoffrey asked. He squeezed her hand. "That's why I married you."

A group of gammies leaped past the window now, and the ghostly octopus followed them with its gammy body, joining the herd.

"Is it a parasite?" Geoffrey wondered.

"Maybe it hunts gammarids like a wolf in sheep's clothing," Nell said.

"Does it kill its prey or just ride it like a bicycle while it's paralyzed?"

"Or does it do both-and trade gammy bodies for new ones when it's through, like a predatory hermit crab?"

"Maybe it lays eggs inside the gammy?"

"Why?" she asked.

"It could be the only place its offspring won't get eaten by other gammies, at least until they gestate. Maybe it moves among them while its eggs hatch and eat the gammarid's insides, and when the offspring are big enough, they come out to catch a ride of their own. It could be something entirely new, honey."

"Well," Nell allowed, "most animals on Earth have many parasites that live in and on their bodies. Nearly a thousand species live only inside the human mouth."

"Yuck!" said Sasha.

"Bacteria, viruses, fungi, and single-celled microbes outnumber cells in the human body by ten to one, and there are ten trillion cells in the human body. We're all walking ecosystems."

"But I've never seen this kind of relationship before. Yes, each of us is an ecosystem that makes up one superindividual," agreed Geoffrey. "But have you ever seen a parasite that climbed on board and turned its vegetarian host into a hunting machine?" Geoffrey asked.

"Yes," Nell said. "We turned horses into hunters and engines of war."

"Oh." He nodded. "Right."

"That's why you married me."

"Uh-huh."

"You guys!" Sasha shouted, rolling her eyes. "You make me sick when you do that! Look!"

A giant gammarid the size of a lion scrabbled on six long legs over an outcrop ten yards below the window.

"A soldier gammy!" Geoffrey whispered.

"It's like the monster in my apartment in New York when I was an undergraduate," Nell said.

"Huh?" Sasha wondered.

"I killed a roach that big with a butcher knife one night, I swear."

"Oh, Nell," Sasha said. "That's not true!"

The soldier gammarid flexed giant mandibles and was covered with spikes pointing laterally in each direction. It was surrounded by much smaller specimens with smaller mandibles that scrabbled underneath it. Then Geoffrey noticed the rippling muscles on the soldier's back. "It's a ghost!"

The possessed gammarid suddenly attacked the smaller amphipods around it with its zombie legs and mandibles, controlled by the mollusk on its back, which fed them into the gammy's mouth.

"Well, that answers it," Nell said. "They're predators that probably lay eggs inside their prey and feed the brood when it hatches inside the exoskeleton."

"Parasite-predators," Geoffrey said.

"Parators?" Nell suggested. "Parasites that parrot their prey."

"Perfect." Geoffrey nodded.

"You guys like naming things, too!" Sasha said. "I still call them ghosts."

"That's actually a really good name, Sasha," Nell said. "Ghost octopus." She shuddered. "You were right. This place is haunted."

With a violent flash of light, they were left in sudden, silent darkness before the glowing creatures of Pandemonium.

Sasha screamed.

"The power went out!" Nell whispered.

Geoffrey looked around, waiting.

After a beat, they heard an engine kick-start and chug somewhere below. The lights came back on, at less than half strength.

"The emergency generator," Geoffrey said.

6:04 A.M.

Maxim continued to key in passwords Sasha may have used to access the door controls. He could try only five before he was locked out and had to wait half an hour before trying again.

The stuffy dormitory was strewn with empty cans of tuna and pineapple, their subsistence for these last days. They had kept watch through the city's cameras, surveying the city and the train station for any sign of entry by humans.

Suddenly, the lights went out.

"Chief!" shouted one of his guards.

"They cut off our power," said Dimitri.

A generator kicked on, throbbing distantly through the floors below, and the lights came back on.

"They're here," Maxim said, grimly.

6:05 A.M.

Nastia spoke through her headset to the others inside the noisy helicopter as they approached Mount Kazar.

"Cold War American complexes like Mount Weather and NORAD's Cheyenne Mountain are dwarfed by the projects of the former Soviet Union," Nastia explained to Ferrell and Jackson, who stood with her behind the cockpit. "Five percent of the population in each republic of the USSR was provided with subsurface accommodations in the event of disaster, though few such facilities were ever finished or adequately supplied." She fired off factoids nervously as they choppered up the snow-patched slopes.

"Hey, let me guess. Did you write a book about this?" Jackson said.

Nastia turned to him seriously and then laughed. "Yes. It's called The Underground History of Planet Earth."

"I'll be sure to buy it at Barnes and Noble, ma'am." He winked. "I must say, you're very pretty for a bookworm. No offense, there, Dima."

Dima had, in fact, tensed as Jackson flirted with Nastia. He tensed more as Nastia noticed it now.

"You're very smart for a soldier," she replied wryly. "No offense."

"I know," Jackson replied.

"All right, we're touching down, folks," Ferrell announced sharply.

The olive drab CH-47 Chinook set down like a locust on a field beside a knoll on Mount Kazar's foothills. The government official sitting up front pointed to a hatch that resembled a closed eyelid. A cracked concrete ramp led down to the rusty hatch.

"That's the entrance you will use to get into the city," said the Kaziristani official in the helicopter. "We cut off power a few minutes ago."