Found inside a larger pyramid known as the Grand Temple at the site of the Aztec city of Tlateloco, the discovery of the new pyramid could push back the age of the city, as well as of its nearby “twin city” Tenochtitlán-by a century or more.

Earlier interpretations of Aztec legends say Tlatelolco was built around AD 1358, the same year as Tenochtitlán, although archeological evidence has cast doubt on that date in recent years.

According to a report in National Geographic News, Guilliem, who is leading an archeological effort to study Tlatelolco, has said that the structure’s construction suggests it could have been built as early as AD 1100 or 1200, at least a century earlier than historical accounts suggest the city was founded.

“If true, the date of this pyramid fits with many other archeological finds that reveal evidence of Aztec occupation earlier than the traditional dates,” said Susan Gillespie, a University of Florida anthropologist.

According to the report, while Guilliem’s team continues to work on determining the new pyramid’s age, the researchers have already uncovered new insights into the Grand Temple.

“Until now we thought Tlatelolco’s Grand Temple had seven phases of construction,” Guilliem told National Geographic News. “Now we know that there are eight,” he added.

As to what’s within the newly discovered pyramid, the research team believes that an offering to Tezcatlipoca Black, the Aztec god of commerce, will be found inside. According to Guilliem, workers intentionally broke into the smaller pyramid in 1368 while building a subsequent phase and deposited a deity that’s likely to be Tezcatlipoca.