One of the fabled "lost cities" of the Andes has been found.

U.S. archeologists who hacked their way for five days into the misty rain forest of Peru's towering mountains have located the well-preserved ruins of a pre-Incan settlement that is believed to date back to about 500 A.D.

Architectural and artistic styles suggest that the culture, previously unknown, flourished for about 1,000 years until it was incorporated into the vast Inca empire and then collapsed with the Incas.

The cause of the Inca empire's collapse, around 1530 A.D., long has been a mystery. Archeologists say the new site could help explain the cause.

Preliminary inspection of the site, in an uninhabited region of northeastern Peru, already has turned up a cluster of 18 two-story stone buildings decorated with stone carvings, agriculturally terraced mountainsides that may extend over hundreds of square miles, stone-paved roads and raised causeways that disappear straight into the jungle, and intact tombs built in niches on the face of a sheer 1,000-foot cliff.

"It's the perfect situation for an archeologist to encounter," said Thomas J. Lennon, one of two leaders of the expedition from the University of Colorado at Boulder that found the site. "The preservation is just excellent. There's been very little disturbance over the years."

Lennon and co-leader Jane C. Wheeler, also at Boulder, said they plan expeditions to the site each summer for the next several years.

"One of the major questions is 'what happened?' " Lennon said. "We think there's a good chance we'll find mummies in the tombs and they may tell us whether there were epidemics that swept through the area."

A leading guess about the demise of the Inca empire is that it was obliterated by diseases introduced by Spanish conquistadores in the 16th century. Measles and smallpox probably were unknown in the New World until Europeans introduced them. Among peoples with no prior exposure, the infections can be fatal.

Legends of "lost cities" in the mountains have flourished for centuries in the Peruvian lowlands, which are dotted with the ruins of many settlements dating back thousands of years.

The site, Lennon said, "has been the subject of rumors and unsuccessful expeditions since the beginning of this century, if not from the time of the Spanish conquest."

The site, called Gran Pajaten, was first located in 1963 when Peruvian farmers wandered into the area. Their report prompted a brief visit by local archeologists. But because the site is so hard to reach, no detailed study was undertaken.

It lapsed back into obscurity until 1983 when a Boulder plastic surgeon, Alan Stormo, was touring the region, heard the legend and learned that the settlement had been found but not studied.

He told a friend at the university and, within months, Lennon and Wheeler had negotiated a contract with the Peruvian government to study the site for five years.

Last summer the Boulder group made their first visit.

The last leg was a five-day trek up the mountain slopes with 10 porters and 10 mules.

The region is so remote, the peaks and rivers have no names.

"When you walk in," Lennon said, "you walk off the map."

"The porters cut through the jungle with machetes," Stormo said. "We climbed under trees, over trees, and through trees."

When they reached the ruins -- at an altitude of 8,600 feet where the ainfall amounts to 60 to 90 inches a year -- the site was overgrown with vines and trees but enough could be pulled away to reveal parts of the buildings.

There were 16 cylindrical houses, each two stories tall, constructed of mortared stone. Set into the outside walls were carved stone heads fringed with stones simulating elaborate feather headdresses.

There also were two rectangular buildings. Pre-Incan mosaics decorated the walls of five houses.

Lennon said the group found lots of pottery, including intact vessels looking as if they hadn't been disturbed in centuries. Some of the pottery styles are previously unknown. Others show Incan influences.

Refuse around the buildings suggests they were living quarters. There were, for example, animal bones showing knife marks. They were bones of bear, jaguar, puma and camel-like creatures that probably were llamas, the domestic animals that supply meat, milk, wool and transport in much of Peru to this day.

"I think we're looking at just a small part of what must have been a very large population," Lennon said. One clue is that zoological expeditions in the area, now part of Peru's new Rio Abiseo National Park, indicate ancient roads and farming terraces covering some 250 square miles.

Not far from the houses the expedition found tombs built into a cliffside.

Some tombs -- small slate-roofed, round towers -- still were sealed and are believed to contain human remains that can be studied by pathologists for signs of disease.

Hanging from the eaves of some tombs were three-foot wooden carvings that Lennon described as "assertively male."

Some of the burial towers are so well preserved that the paint still is on them.

"This place is so rich," Lennon said, "there's no doubt in my mind we're going to come up with all sorts of new insights about these ancient people."