Tlaloc, the god of rain and fertility, was greatly feared among the Aztec, who drowned children to appease him.
(See a related National Geographic magazine feature on "Mexico's Pyramid of Death.")
"This is another fabulous discovery from the Great Temple precinct, and there are bound to be many more buried objects yet unearthed," said Susan Gillespie, an Aztec expert at the University of Florida in Gainesville.
"What is significant about this find is the early date of the altar frieze, evidenced by the cruder style of the bas-relief compared to the many late Aztec sculptures that have been recovered," she added.
"With such finds archaeologists can begin to more firmly trace the changes in state-sponsored religious practices at the Great Temple."
(Read related story: "Ancient Pyramid Found at Mexico City Christian Site" [April 2006].)
The giant monolith, meanwhile, is believed to be standing in its original position. The rectangular piece is still partly buried, and archaeologists can only see one of its sides.
López Luján estimates that the stone, which he says comes from the Chiquihuite stone formation north of Mexico City, could weigh as much as 12 tons (11 metric tons).
The monolith corresponds to the last phase of the Aztec empire, from 1487 to 1520.
"It is a typical monument of Aztec imperial style," López Luján said.
The upper face of the monolith has deep carvings.
"Taking into account its position, the form, and what I can see from a side, it should represent the Earth God (Tlaltecuhtli), the Earth Goddess (Tlaltecuhtli, Coatlicue), or a nocturnal deity such as Itzpaplotl of Coatlicue," López Luján said.
Some archaeologists speculate it could lead into an underground chamber.
"The importance of the monolith is what we are going to discover," Alberto Diaz, a member of the archeological team, told the Reuters news agency.
"It's likely that it is part of a chamber, of some offering. We won't know until we get close. First we have to get the stone out."
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