A story passed down for generations says Amecameca once stood on another site, and was abandoned after an eruption of the Popocatepetl volcano. Residents suspect the ruins, believed to date from A.D. 700 to 1,100 and are on the outskirts of the present-day settlement, could help clarify that matter.
"This represents a possibility for the people to recover that part of Amecameca's history," said activist Rebeca Lopez Reyes, of the local preservationist group Guardians of the Volcanos.
The ruins detected so far in Amecameca are not particularly spectacular. Only about 120 square yards of the estimated 5-acre site have been excavated, revealing stone and clay footings for houses that may have supported upper walls of wood or clay wattle. But the very ordinariness may mean the site is significant.
"What makes this important is that it is a residential area, not a ceremonial or religious site," said Felipe Echenique, a historian who leads the academic workers' union for the National Institute of Anthropology and History, or INAH, which is in charge of reviewing the site.
"In Mexico, we really have very little evidence of how the cities really were, or how people lived," said Echenique, who was not involved in the dig but is familiar with preliminary findings.
The housing compounds were apparently built by one of the still-unnamed cultures living in the Valley of Mexico long before the Aztecs appeared in the area in 1325 and founded Tenochtitlan, the precursor to Mexico City.
Lopez Reyes said researchers called in by the INAH to investigate the site of the proposed roadway have found ceramic pots, bones, and a stone serpent's head, suggesting that the god Quetzacoatl, "the Feathered Serpent," may have been worshipped there centuries before the Aztecs paid him homage.
The few excavations of residential areas carried out so far in Mexico have yielded fascinating details.
In Teotihuacan, one of the biggest pre-Hispanic cities located northeast of Mexico City, some houses appear to have been illuminated by narrow doorways that opened onto central patios with shallow pools that acted as "water mirrors" to direct light inside the rooms. Techniques for building windows were apparently not yet known.
Investigators say similar discoveries could emerge from Amecameca.
The Amecameca protesters have set up a camp to guard against construction work or looters and to explain the ruins to passers-by. They are asking the road be rerouted.
Authorities have not yet commented on the demands, and the builders of the roadway, known as the Mexican Beltway, did not immediately respond to requests for comment.
INAH spokesman Arturo Mendez said that "in almost every project of this type, there are going to be discoveries" of pre-Hispanic material.
The institute normally sends in a rescue project to excavate, recover any significant items, carefully rebury the site for possible future exploration, and then allow the construction to continue.
That is basically what happened in the 1960s to Maya ruins known as Tortuguero in the southern state of Tabasco. It was split in half and largely covered by highway construction.
The site held a stone monolith known as Monument Six, which contains one of only a couple of known references in Mayan glyphs to the date 2012, which some believe marks the end of the Mayan Long Count calendar and a possible apocalypse.
The inscription has become so famous that the Tabasco state government now uses it on advertisements to promote tourism, even though the stone fragment itself sits in a museum in the nearby city of Villahermosa and little is left of the ceremonial site where it was excavated.
The people of Amecameca say they want to prevent that from happening to them.
Maria de los Angeles Eusebio, 55, a retired anthropologist, is one of the residents who have camped out to prevent construction machinery from going through. Equipped with tents, coffee, "and lots and lots of blankets," residents are staying day and night, through wind, rain, and cold, to ensure the remains of their ancestors' city aren't destroyed.
"We don't want them to just bury this and run the highway over the top of it," said Eusebio. "We want them to return the artifacts, so we can display them in a museum for the community."