Archaeologists restoring a 16th century building in the heart of Mexico City have uncovered an eight-ton stone that is testimony to the Aztecs' artistic ability and to their bloody religious rites.

The disk-like monolith, found in what once served as the Spanish archbishop's headquarters across from the National Palace, is a Cuauhxicalli, or eagle's cup, central to the Aztecs ceremony of human sacrifice."Within the Aztec's rite, they sacrificed a warrior and they took out his heart," said archeologist Guillermo Perez Castro.

"When it cooled, they deposited the heart in the open center of the stone," said Perez Castro, head of historical archaeology for the National Institute of Anthropology and History (INAH).

But despite its gruesome use, archaeologists say the 500-year-old stone gives amazing insight into the Aztec's religious, cultural and political life.

"The stone is a representation of sacrifice, of faith and the necessity to feed the gods," Perez Castro said of the well-preserved artifact.

The face of the stone, six feet in diameter, is carved with concentric circles of pearls, sun rays and eagle feathers.

In the center is the partially hollowed receptacle in which lies the carved face of the god of air and fire, known as Xiuhtecutli, who according to the Aztec mythology received the sacrificed heart after it had been presented to the sun, the Aztec's principal god.

The monolith originally sat at the base of a pyramid temple. At the top, an Aztec priest would perform the sacrifice, offering the still beating heart to the sun.

"The sacrifice that they did here was so that the sun would return day after day," Perez Castro said.

On the side of the stone, which is 27.5 inches high, are 11 intricately carved friezes of mythological representations of the Aztecs' conquests of other tribes from central and eastern Mexico.

The Aztecs, who considered their city Tenochitlan the center of the universe, set out to dominate other communities politically, economically as well as spiritually, INAH archaeologist Pedro Francisco Sanchez said.

Thus the friezes, which depict conquests in chronological order, show Aztec gods vanquishing the gods of other tribes.

In what archaeologist have determined is the first of the frieze series, an Aztec figure with attributes of the gods Huitzilopochtli, Xiuhtecutli and Tezcatlipoca is depicted dominating Ciuahcoatl, a deity from the southern Culhuacan tribe.

Ironically, the stone was uncovered while archeologist were excavating to find out what was causing large cracks in the walls of archbishop's headquarters, which now serves as offices of the finance ministry.

It was well known that the Spaniards who conquered the Aztecs in 1521 tore down the temples of Tenochitlan and built Mexico City on top of them.

The archbishop's headquarters was built on the grounds of the Aztecs' main temple, large sections of which have already been excavated.

"We knew that there was a high probability of finding something because of the richness of the place," Perez Castro said.

When the archaeologist began digging last June, they discovered the cracks were caused by the archbishop's building sinking in Mexico City's soft subsoil over remnants of Spanish and Aztec builings.

On July 1, after digging about 10 feet, the archaeologists' picks struck the monolith.