STORY: Parts of Mexico's remote southern jungles have barely changed since the time of ancient Maya.
But now, scientists and environmental activists say the pristine wilderness and ancient cave systems beneath the jungle floor of the Yucatan Peninsula are critically endangered.
It’s due to a new government railway project - the Tren Maya - which aims to bring connectivity and economic benefits to deprived areas.
The 910 mile long rail line is set to connect Mexico's top tourist destination Cancun to the ancient Mayan temples of Chichen Itza and Palenque.
But experts warn the train will disrupt wildlife routes and already fragile ecosystems.
Local guide Ismael Lara shares the concerns.
"Here we suffer from a terrible drought. From April, and May onwards, temperatures reach 104 Fahrenheit and, unfortunately, we don’t have a water supply. These animals have to migrate to other places to find water, above all. The train will split the jungle and will interrupt these animals' way to find water."
President Andres Manuel Lopez Obrador has pledged to finish the flagship rail route by the end of 2023.
But the $20 bn project has divided Mexicans and raised questions of how to best to balance economic progress with environmental responsibility.
Mexico’s tourism agency, charged with the project, says the railway will lift more than a million out of poverty and create up to 715,000 new jobs by 2030.
But it will also bring the modern world closer to vulnerable species, such as jaguars and bats.
The train will pass above a system of thousands of subterranean caves carved out from the region's soft limestone bedrock by water over millions of years.
Scientists and activists say the government has cut corners in environmental risk assessments in an attempt to complete the project while Lopez Obrador is still in office.
One environmentalist told Reuters they spotted construction material leaking into a cave, affecting water supplied to people and animals living on the peninsula.
Part of the government's impact study for the project says the environmental risks are "insignificant" and have been adequately mitigated.
The ancient caves have also been the site of discoveries, such as human fossils and Maya artifacts, like a canoe estimated to be more than 1,000 years old.
Despite the concerns about the railway, it has the support of many in villages, who for generations have been largely forgotten in national development plans.
"I think it’s an ambitious project. Especially because it will bring infrastructure and tourism, which is our main source of economy. Therefore, it will help us quite a lot. If we get to have this project and keep it, we will get the progress we need."