The Mexican government has invoked national security powers to forge ahead with a tourist train along the Caribbean coast that threatens extensive caves where some of the oldest human remains in North America have been discovered.

President Andrés Manuel López Obrador is racing to finish his Maya Train project in the remaining two years of his term over the objections of environmentalists, cave divers and archaeologists.

The government had paused the project earlier this year after activists won a court injunction against the route, because it cut a swath through the jungle for tracks without previously filing an environmental impact statement.

But the government invoked national security powers last week to resume the track laying. López Obrador said the delay had been very costly and the decree would prevent the interests of a few from being put above the general good. In November, his government had issued a broad decree requiring all federal agencies to give automatic approval for any public works project the government deems to be “in the national interest” or to “involve national security.”

Activists say the heavy, high-speed rail project will fragment the coastal jungle and will run often above the roofs of fragile limestone caves known as cenotes, which – because they’re flooded, twisty and often incredibly narrow – can take decades to explore.

Inside those water-filled caves are archaeological sites that have lain undisturbed for millennia.

The cave systems have mainly been through the efforts of volunteer cave divers working hundreds of yards (meters) inside the flooded caverns. Caves along the Caribbean coast have yielded treasures like Naia, the nearly complete skeleton of a young woman who died around 13,000 years ago.

She was discovered in 2007 by divers and cave enthusiasts who were mapping water-filled caverns north of the city of Tulum, where the train line is heading.

“Just in this one stretch of 60 km. (of planned train tracks), there are 1,650 km. of flooded caves full of pure, crystalline water,” said Octavio del Rio, a diver and archaeologist who has been exploring the region for three decades. In 2004, Del Rio himself participated in the discovery and cataloguing of The Woman of Naharon, who died around the same time, or perhaps earlier, than Naia.

“I don’t know what could be more important than this, right?” said Del Rio. “We are talking about the oldest remains on the continent.”

The 1,500- km. Maya Train line will run in a rough loop around the Yucatan peninsula, connecting beach resorts and archaeological sites.

The government’s National Institute of Anthropology and History is tasked with protecting relics along the route, but its experts largely aren’t able to take the deep, long, extended dives needed to reach the flooded caves. Even near the surface, where most of the government’s archaeological work has been done, there have been stunning discoveries along the proposed path of the train.

Government archaeologist Manuel Perez has acknowledged that an almost fully preserved small Mayan temple – complete with wood roofing – has been located in a cave near the train’s path. He has suggested the route be changed.

But his boss, Diego Prieto, the head of the institute, appeared to rule out changing the path of the train, for which workers have already cut down a 50-m.-wide swath of jungle dozens of kilometres long. He suggested most of the relics, in the few months left before the train is built, can simply be picked up and moved.

“The problem isn’t the route… even if the route is changed, there are going to be lots of discoveries anyway,” said Prieto. “The problem is the archaeological work to gather the material found, and conserve those structures that should remain on site.”

The caves along the coast were probably dry 13,000 years ago, during the last ice age, and so once sea levels rose at the end of the ice age and they flooded, they acted as time capsules – very fragile ones. The government’s plan is to sink beams and cement columns through the roofs of the caves, probably collapsing them – and the invaluable relics they hold – to support the railway.

That’s not to mention the 68-km. swath of jungle that is being cut down to make way for this segment of the train line, in addition to the tons of crushed rocked that will have to be piled atop the soil to create a bed for the 160-kph train.

Activists say their efforts are labour of love.

Urbina Bravo, a diver and environmentalist who has worked on the Caribbean coast for decades, said “making decisions without the support of science, without the backing of specialists has cost us very dearly” in projects around the world. “We continue and will continue to pay the price for these errors.”

But López Obrador dismisses critics as “pseudo-environmentalists” acting on behalf of business interests or political opponents. The president attacks experts, activists and anyone who questions his sudden and unplanned decision to run the railway line through the jungle, which he dismissively calls “acahual,” (roughly, ‘second-growth forest’).

Fernando Vázquez, the spokesman for the government tourism agency building the train line, says “there are people who in essence are not necessarily working in favour of the environment, but rather are specifically activists against the Maya Train.”