Mexico City ? Mexican archaeologists using ground-penetrating radar have detected underground chambers they believe contain the remains of Emperor Ahuizotl, who ruled the Aztecs when Columbus landed in the New World. It would be the first tomb of an Aztec ruler ever found.

The find could provide an extraordinary window into Aztec civilization at its apogee. Ahuizotl (ah-WEE-zoh-tuhl), an empire-builder who extended the Aztecs’ reach as far as Guatemala, was the last emperor to complete his rule before the Spanish Conquest.

Accounts written by Spanish priests suggest the area was used by the Aztecs to cremate and bury their rulers. But no tomb of an Aztec ruler has ever been found, in part because the Spanish conquerors built their own city atop the Aztec’s ceremonial center, leaving behind colonial structures too historically valuable to remove for excavations.

One of those colonial buildings was so damaged in a 1985 earthquake that it had to be torn down, eventually giving experts their first chance to examine the site off Mexico City’s Zocalo plaza, between the Metropolitan Cathedral and the ruins of the Templo Mayor pyramid.

Archaeologists said they have located what appears to be a 6-foot-by-6-foot entryway into the tomb about 15 feet below ground. The passage is filled with water, rocks and mud, forcing workers to dig delicately while suspended from slings. Pumps work to keep the water level down.

“We are doing it very, very slowly … because the responsibility is very great and we want to register everything,” said Leonardo Lopez Lujan, the lead government archaeologist on the project. “It’s a totally new situation for us, and we don’t know exactly what it will be like down there.”