“What makes everything glow so brightly?” Schroeder asked as he limped along a street with Karla at his side.

“I studied light-emitting minerals as part of a geology course,” Karla said. “Some minerals glow under the influence of ultraviolet rays. Other types emit light from radiation or chemical change. But if we're right, and this is an old volcano, maybe there's a thermoluminescent effect caused by heat.”

“Could this be an old magma chamber?” Schroeder said.

“That's possible. I just don't know. But there's one thing that I'm absolutely sure of.”

“What's that, my dear?”

She gazed with awe at the glimmering edifices that stretched off in every direction. “We are strangers in a strange land.”

After leaving the mural tunnel that led to the city, they had walked under a huge corbel arch down a broad ramp to an open plaza with a step pyramid built of huge blocks at the center. The processional motif, including the domesticated woolly mammoths, was continued on the exterior levels of the pyramid, although the colors were less bright than in the access tunnel. Karla surmised that it was a temple or platform for priests or speakers to address people gathered in the plaza.

A paved boulevard about fifty feet wide led into the heart of the city. They had strolled along the thoroughfare like a couple of tourists bedazzled by the bright lights of Broadway. The buildings were much smaller than Manhattan's skyscrapers—three stories at the most—yet they were architectural wonders, considering their probable age.

The promenade was lined with pedestals. The statues they once supported lay in unrecognizable heaps of rubble, as if they had been pushed off their perches by vandals.

Schroeder rested his sore ankle, then he and Karla explored a couple of buildings, but they were as empty as if they had been swept clean with a big broom.

“How old do you think this place is?” Schroeder said as they plunged deeper into the city.

“Each time I try to date it, I become tangled in contradictions. The fact that the murals show humans and mammoths coexisting places them in the Pleistocene period. That was a time span that ran from 1.8 million to ten thousand years ago. Even if we go with the most recent date of ten thousand years, the high level of civilization here is astonishing. We've always assumed that mankind didn't evolve from its primitive state until much later. The Egyptians' civilization is only around five thousand years old.”

“Who do you think built this wonderful city?” '

“Ancient Siberians. This island was connected to an arctic continental shelf that extended out from the mainland. I didn't see any pictures of boats, indicating that this was pretty much a landlocked society. From the looks of it, this was a rich city.”

“Since it was such a flourishing society, why did it end?”