Rays of sunlight slice through pools of crystal water as clusters of fish cast shadows on the limestone below. Arching over the emerald basin are walls of stalactites dripping down the cavern ceiling, which opens to a dense jungle.

These glowing sinkhole lakes — known as cenotes — are a part of one of Mexico’s natural wonders: A fragile system of an estimated 10,000 subterranean caverns, rivers and lakes that wind almost surreptitiously beneath Mexico’s southern Yucatan peninsula.

Now, construction of President Andrés Manuel López Obrador’s crown jewel project — the Maya Train — is rapidly destroying part of that hidden underground world, already under threat by development and mass tourism. As the caverns are thrust into the spotlight in the lead-up to the country’s presidential elections, scientists and environmentalists warn that the train will mean long-term environmental ruin.