Researchers found a tunnel under the Temple of the Snake in the pre-Hispanic city of Teotihuacan, about 28 miles northeast of Mexico City.
The tunnel had apparently been sealed off around 1,800 years ago.
Researchers of Mexico's National University made the finding with a radar device. Closer study revealed a "representation of the underworld," in the words of archaeologist Sergio Gomez Chavez, of Mexico's National Institute of Anthropology and History.
Experts found "a route of symbols, whose conclusion appears to lie in the funeral chambers at the end of the tunnel."
The structure is 15 yards beneath the ground, and it runs eastwards. It is about 130 yards long.
"At the end, there are several chambers which could hold the remains of the rulers of that Mesoamerican civilization. If confirmed, it will be one of the most important archaeological discoveries of the 21st century on a global scale," Gomez Chavez said late Thursday.
Teotihuacan, with its huge pyramids of the Sun and the Moon, its palaces, temples, homes, workshops, markets and avenues, is the largest pre-Hispanic city in Mesoamerica. It reached its zenith in the years 300-600 AD.
Explore further: Dinosaur no more: UK museum's Dippy to be retired in 2017