“Maybe it didn't end. Maybe it simply moved somewhere and became the basis of another society. There is evidence that Europeans as well as Asians populated North America.”

As Schroeder mulled the implications of Karla's analysis, excited voices could be heard shouting from behind them in the direction of the city gate. He squinted back along the boulevard. Pinpoints of light were moving in the area around the plaza. The ivory hunters had also blundered into the city.

“We're sitting ducks out here in the open,” Schroeder said. “We can lose them easily if we get off this lovely avenue.”

He slipped into an alley that connected with a narrow side street. The buildings were smaller than on the main boulevard; none was taller than one story. They appeared to serve more of a residential function than the grander, more ceremonial structures lining the main drag.

As a former soldier, Schroeder had accurately sized up the defensive situation. The city was a huge maze of hundreds of streets. Even with the omnipresent halo of light that shimmered over the city, as long as they kept alert and on the move through the labyrinth their pursuers would never catch them. At the same time, Schroeder was aware that they could only run for so long. Eventually, they would run out of food and water. Or luck.

His goal was to get to the far side of the city. He had hopes, supported by the relatively good quality of the air, that there might be another way out. The people who built this subterranean metropolis seemed to have done so with logic and reason. Thus it would be logical and reasonable that they had more than one way to get in and out. They were more than halfway across the city when Karla cried out in alarm.

She dug her fingers into Schroeder's arm. He slipped the automatic rifle off his shoulder. “What is it?” He glanced around at the silent façades as if he expected to see the leering faces of the ivory hunters in the windows.

“Something ran down that alley.”

Schroeder followed her pointing finger with his eyes. Although the buildings produced their own light, they were built close together, and the narrow spaces between them were in deep shadow.

“Something or someone?”

“I—I don't know.” She laughed. “Maybe I've been underground too long.”

Schroeder had always trusted his senses above his analytical skills. “Wait here,” he said. He approached the alleyway with his finger tight on the trigger. He edged up to the alley, stuck his head around the corner and flicked the flashlight on. After a few seconds, he turned and came back. “Nothing,” he said.

“Sorry. It must have been my imagination.”

“Come,” he said, and, to Karla's surprise, he headed toward the alley.

“Where are you going?”

“If there is something out there, it's better that we sneak up on it rather than the other way around.”