The world’s largest cave, the mammoth Hang Son Doong in Vietnam, is a relative babe-in-arms when it comes to natural history: The cavern was first discovered in 1991, then lost, then found again before it was first explored in 2009. But less than a decade later, environmentalists are scrambling to save the site from thousands of tourists and a development company set on thrusting a cable car into its depths.

Located in Phong Nha-Ke Bang national park, a UNESCO World Heritage Site, Son Doong stretches more than 5.5 miles underground, reaches heights of 650 feet and is home to its own jungle, ecosystem and river. Just one tour company has a concession to venture into the cave, and only a few hundred people are allowed inside the fragile environment every year. But that could soon change.

HuffPost RYOT spoke with local activists, cavers and the spelunkers who first explored Son Doong about the local government’s secretive plans that may allow developers to build a massive cable car project that’d bring more than 1,000 people to the cave each day.

Such threats are not new.

In 2014, a group of local activists helped stave off a proposal to build a $212 million, 6.5 mile-long cable car throughout Phong Nha-Ke Bang park that would traverse parts of Son Doong. The announcement drew widespread condemnation despite assurances from the cable car company, Sun Group, that it would be an environmentally friendly way to view the park that’d bring “thousands of jobs for the poor local people.”

Any plans at Son Doong would likely echo another project, completed by another company last year, at the country’s Mount Fansipan, one of the tallest peaks in the region. The site is now home to a cable car that cut a journey that used to take three days on foot down to 20 minutes. The system can now bring 2,000 people to the summit every hour, and, like the country’s iconic Ha Long Bay, has become a hive of tourist activity.

Jason Speth/HuffPost RYOT Despite its size, Son Doong wasn't discovered until 1991. It was lost again for nearly two decades and was fully explored for the first time in 2009.

Environmentalists fear that if the Son Doong project were to go forward, it would bring hordes of tourists to a pristine ecosystem protected by its relative unreachability.

A Facebook group operating under the name Save Son Doong has called on the Vietnamese government to again reject the proposal, saying the “project must be stopped before it has a huge and damaging impact on the Son Doong cave and the ecosystem of the entire Phong Nha-Ke Bang national park,” a Change.org petition organized by the group says.

Those concerns were echoed by the British Cave Research Association, the group that declared Son Doong the largest of its kind and, alongside the explorer Howard Limbert, led the first expedition to the site.

Jason Speth/HuffPost RYOT Vietnam's Son Doong cave, the largest in the world, could hold a 40-story skyscraper inside. The pristine ecosystem has its own river and jungle.

“Caves are fragile environments. They don’t regenerate very quickly, and once they’re broken, they’re broken,” Deb Limbert, a team leader for the group, told HuffPost RYOT.

Government officials in the province of Quang Binh, where Son Doong is located, have told local media there are no plans to support a new cable car project.

In March, HuffPost RYOT filmed a representative of the cable car company traveling to the cave with Phong Nha-Ke Bang park employees.

Take a look at more photos of Son Doong cave below.

Jason Speth/HuffPost RYOT Environmentalists worry a cable car would damage the cave's fragile ecosystem.

Jason Speth/HuffPost RYOT An underground river helped carve out the Son Doong cave over millions of years.

Jason Speth/HuffPost RYOT The cave has been protected from the masses by its relative inaccessibility.

CORRECTION: An earlier version of this article included two pictures of Hang En cave instead of Son Doong.