In 2009, a team from the British Cave Research Association trekked to the Annamite Mountains in eastern Vietnam, near the Laos border, to map Hang Son Doong (“Mountain River Cave”). What they found shattered all records for largeness. An entire block of New York City skyscrapers could fit comfortably within the enormous cavern.

Over five kilometers long, more than six hundred feet deep, and almost five hundred feet wide, the interior of Hang Son Doong is large enough to host its own ecosystems. An entire rain forest grows in one section of amphitheater, fed by a solitary shaft of sunlight from above and the enormous underground river that gives the cave its local name. In the towering column of air above the cave floor, whispery licks of cloud drift in the still air.

More than 150 caves dotting the mountains surrounding Son Doong make up Phong Na-Ke Bang National Park. Caves are legendary features in Vietnam’s national psyche. Ho Chi Minh’s resistance against the French colonists was planned in a cave. Underground tunnels and passages served as bomb shelters and hidden passages during the war against America. To the Vietnamese, caves represent the resilience of their national character, and visitors arrive in droves to see the park’s namesake Phong Nha cave, which gets lit up like a rock concert for people to walk around in.

Local men told the British spelunking team that they had happened upon Son Doong cave in the early 1990s, but had not explored the depths of the cavity due to the fearsome whistling noise the underground river made when amplified by a six hundred foot tall limestone microphone. The skinny eastern belly of Vietnam, as well as much of central Laos, is carved out of an immense limestone block, which was squeezed upwards out of the ocean roughly fifty million years ago, when the Indian subcontinent decided it wanted to get cozy with the rest of Asia. Across the region, rugged limestone karst formations scratch at the sky like enormous gray teeth, forming some of Southeast Asia’s most iconic geological features.

When the British team navigated through Son Doong’s underbelly, they were forced to turn back when they came face to face with a 200 foot high wall of sheer slick mud. For almost two years, a complete survey of the cave was impossible. As Uncle Ho once said, “When the prison doors are opened, the real dragon will fly out.”