have to find that source and either mine it ourselves or
close it off. As yet we're still trying to find it."

Ross shook his head. "It must be a long way back. Those guys who
discover tombs and dig up old cities--couldn't they give you some hints?
Wouldn't a civilization like that have left something we could find
today?"

"It depends," Ashe remarked, "upon the type of civilization. The
Egyptians built in stone, grandly. They used tools and weapons of
copper, bronze, and stone, and they were considerate enough to operate
in a dry climate which preserved relics well. The cities of the Fertile
Crescent built in mud brick and used stone, copper, and bronze tools.
They also chose a portion of the world where climate was a factor in
keeping their memory green.

"The Greeks built in stone, wrote their books, kept their history to
bequeath it to their successors, and so did the Romans. And on this side
of the ocean the Incas, the Mayas, the unknown races before them, and
the Aztecs of Mexico all built in stone and worked in metal. And stone
and metal survive. But what if there had been an early people who used
plastics and brittle alloys, who had no desire to build permanent
buildings, whose tools and artifacts were meant to wear out quickly,
perhaps for economic reasons? What would they leave us--considering,
perhaps, that an ice age had intervened between their time and ours,
with glaciers to grind into dust what little they did possess?

"There is evidence that the poles of our world have changed and that
this northern region was once close to being tropical. Any catastrophe
violent enough to bring about a switch in the poles of this planet might
well have wiped out all traces of a civilization, no matter how
superior. We have good reason to believe that such a people must have
existed, but we must find them.

"And Ashe is a convert from the skeptics--" the major slipped down from
his perch on the wall shelf--"he is an archaeologist, one of your tomb
discoverers, and knows what he is talking about. We must do our hunting
in time earlier than the first pyramid, earlier than the first group of
farmers who settled by the Tigris River. But we have to let the enemy
guide us to it. That's where you come in."

"Why me?"

"That is a question to which our psychologists are still trying to find
the answer, my young friend. It seems that the majority of the people of
the several nations linked together in this project have be