EVOLUTIONARY EDIFICE. Ancient residents of present day Mexico built the first pyramid on this site in about 100 AD. Then these people, who preceeded the Toltecs and Aztecs, began renovating. Archaeologists have now identifed the remains of five successive pyramids--each encasing the previous one---built over the next several hundred years. Recently discovered burial sites inside the pyramid are providing clues to this vanished civilization.

Between 100 and 500 AD, an ancient people built a thriving metropolis called Teotihuacan on a plateau about 25 miles from present-day Mexico City. With its precisely aligned avenues and a huge plaza surrounded by 15 monumental pyramids, Teotihuacan was grander than any city in Europe at the time. It sprawled over nearly 8 square miles and was inhabited by 200,000 people. Then, 700 years before the Aztec's began constructing their capital city of Tenochtitlan, this ancient civilization--contemporary with that of ancient Rome--just vanished.

Now, archaeologists excavating inside the oldest monument on the site--the Pyramid of the Moon--are unearthing clues to the history of this mysterious culture. Tunnels dug into the structure have revealed that the Teotihuacan's didn't remain content with their architectural feats for very long. Over a period of several hundred years, the pyramid underwent at least six facelifts, and each new addition was larger and covered the earlier structure. The first pyramid was built from small bricks around 100 AD. The outermost layer--consisting of large stone blocks--matches the style of later pyramids on the site, which were built in a single stage. The current pyramid is 151 feet high and its walls are precisely aligned with the walls of every other structure in the city.

BONES AND BAUBLES. Excavation reveals leg bones of sacrificial victims lying side by side. The victims were dressed with shell beads and other decorations. Large conch shells mark the corners and sides of the burial pit.

As the investigators have burrowed through the layers of the pyramid, they have discovered artifacts that are beginning to provide a timeline to the history of Teotihuacan. The latest find, made by a team led by Saburo Sugiyama, associate professor at Aichi Prefectural University in Japan and adjunct faculty at Arizona State University, and Ruben Cabrera of Mexico's National Institute of Anthropology and History, is a tomb apparently made to dedicate the fifth phase of construction. It contains four human skeletons, animal bones, jewelry, obsidian blades and a wide variety of other offerings. The corners of the burial pit are marked with large conch shells. The tentative date for this burial is between 100 and 200 AD.

Another dedicatory tomb discovered a year ago was associated with the fourth stage of construction. It contained only one human--a bound, male sacrificial victim--as well as wolf, jaguar, puma, serpent and bird skeletons, and more than 400 other offerings, including large greenstone and obsidian figurines, ceremonial knives and spearpoints. "The contents of this new burial appear to be significantly different," says Sugiyama, who notes that they more closely resemble burials discovered under another monument on the site, the Feathered Serpent Pyramid.

For example, the obsidian blades in the new burial are green--a color of obsidian lacking in the tomb in pyramid four, but common in the Feathered Serpent burials. The tomb also contained a greenstone "butterfly" nose pendent that is "exactly the same style as the ones we found at the Feathered Serpent Pyramid," says Sugiyama. There are also more military items among the offerings, and a larger number of human sacrifices, both similar to the Feathered Serpent burials, where researchers excavated more than 130 human skeletons, most of them soldiers who may have been war captives.

The excavations also reveal a major jump in size and complexity occurring with the construction of pyramid four. It underwent a major remodeling that altered its alignment to conform with the city's precise grid structure. In addition, the remodeling involved the first use on the Pyramid of the Moon of the "talud-tablero" architectural style, common to other monuments on the site.

PONDERING. Archaeologist Saburo Sugiyama looks for clues about a vanished civilization in a 2,000 year-old sacrificial grave.

As the digging proceeds, Sugiyama expects to locate additional burial sites, especially in the early versions of the pyramid, where primitive loose rock construction may have discouraged looters. And he believes he knows where to look next. "We have noticed that this tomb is a few feet to the east of the city's north-south axis," said Sugiyama. "These people were generally very precise and they rarely did anything unsymetrically."

But whatever the team discovers about the Teotihuacan's culture, it may not solve the biggest mystery of all--why this 2,000 year old civilization disappeared. What may have been the largest metropolis in the Americas was already a ghost town by the time the Aztecs arrived. They gave its great buildings, boulevards and plazas their present names but, they never moved in. The Aztecs called it the "Place of the Gods," where, they believed, the world was created.

Images: JANET MONTOYA, Arizona State University