A Hidden Giant in Vietnam
Deep in Phong Nha–Kẻ Bàng National Park in Quảng Bình, Vietnam, lies Hang Sơn Đoòng, the world’s largest known natural cave. Discovered in 1991 by local woodsman Hồ Khanh and scientifically surveyed in 2009 by the British Royal Cave Association, the cave remained unexplored for millions of years until its vast dimensions were revealed to the world.
Record-Breaking Scale
Stretching nearly 9 kilometers (5.6 miles) long, Sơn Đoòng’s chambers are immense — up to 150 meters (492 feet) wide and over 200 meters (656 feet) high. Its estimated volume reaches 38.5 million cubic meters (1.36 billion cubic feet), enough to fit 15,000 Olympic-sized swimming pools.
Some sections are so massive that a 40-story skyscraper could stand upright inside without touching the ceiling. Within its depths are stalagmites up to 70 meters (230 feet) tall, a 2.5-kilometer (1.55-mile) underground river, coral fossils, and formations dating back millions of years.
A Cave With Its Own Weather
Sơn Đoòng’s vast interior creates a unique microclimate. Two enormous roof collapses, called giếng trời (“skylights”), allow sunlight and air to enter, triggering the formation of clouds inside the cave.
When warm, humid air from the underground river rises and meets cooler airflow from outside, condensation creates mist and occasional rain. These skylights also support lush vegetation, forming underground jungles such as the famous “Garden of Eden.”
Preservation and Exploration
Photos captured by Carsten Peter in 2010 revealed Sơn Đoòng’s extraordinary scale to the world, drawing international attention. In 2013, limited tourism began under strict regulation, managed by Oxalis Adventure, allowing only about 1,000 visitors per year.
Treks last four days and three nights, combining camping inside the cave with scientific discovery. Authorities have restricted large-scale development projects, such as proposed cable cars, to protect Sơn Đoòng’s fragile ecosystem and rare microclimate.
Hidden deep in Vietnam’s Phong Nha–Kẻ Bàng National Park, Hang Sơn Đoòng is the world’s largest known cave.
Discovered in 1991 by Hồ Khanh and surveyed in 2009, its massive size, underground river, and even its own weather make it extraordinary…🧵👇 pic.twitter.com/bxWCePPnha
— Fascinating True Stories (@FascinatingTrue) August 20, 2025