Experts in Mexico have detected thousands of buried ancient ruins and artifacts. The discoveries along the route of the “Maya Train” project could slow down the proposed plan—which opponents say threatens native peoples and water supplies.

This summer, President Andrés Manuel López Obrador launched the start of construction on the Maya Train, his pet project. If built, it would run some 950 miles in a rough loop around the Mexican state of Yucatán.

The train is supposed to connect Caribbean beach resorts to the Yucatán peninsula’s mostly native populations and historic ruin sites. Officials hope to stimulate economic development near the train’s 15 stations. The government says the train will cost as much as $6.8 billion. Others say it will be much more.

Critics contend that López Obrador rammed the project through approvals. They say there wasn’t enough study of its effects on the environment, underground sinkhole caves, and historic sites.

LiDAR, which stands for Light Detection and Ranging, involves shooting a pulsed laser at the ground. The technology can give a detailed image of the surface, even through dense vegetation. LiDAR data shows 2,187 “archaeological monuments” along 277 miles of the proposed train route. Experts already knew about the existence of some of the sites, but some are new discoveries.

The term monuments can mean many things: the ruins of a pre-Hispanic Maya home, carved stones, the remains of temple platforms, or other artifacts. It’s not clear how many of each type LiDAR has detected. But experts at Mexico’s National Institute of Anthropology and History say at least 91 are large-scale structures like plazas and pyramid or temple platforms.

Institute officials say that the builders of the train must take “specific measures” to avoid damaging the artifacts—even though some were probably disturbed by railway construction decades ago. Those officials have not said whether they believe parts of the train route will need to be rerouted.

The Mayas formed a sprawling empire of city-states across the Yucatán and Central America between 2,000 B.C. and A.D. 900. Their descendants still live on the peninsula.

Some Mayan communities have filed court challenges against the Maya Train project. They argue that it will cause environmental damage. They also say they were not widely consulted about the plan—or that they won’t share in its benefits.

(Tourists walk at the Mayan ruins of Chichen Itza in Mexico’s Yucatán Peninsula in this August 2018 file photo. AP Photo/Eduardo Verdugo)