The world's largest cave passage could fit a 40-story skyscraper inside. It has its own jungle, river, and clouds that form beneath the ceiling when warm and cool air meet.
Doug Knuth from Woodstock, IL, CC BY-SA 2.0, via Wikimedia Commons
A Subterranean World
Hang Son Doong is the largest known cave passage on Earth by volume. The main passage extends for more than 5 kilometers, with sections reaching 200 meters in height and 150 meters in width. Its total measured volume is 38.5 million cubic meters. The cave formed between two and five million years ago within a 400-million-year-old block of limestone in the Truong Son mountains. A subterranean river, the Rao Thuong, carved out this immense tunnel by following a major geological fault line.
The entrance was first seen in 1990 by a local man, Ho Khanh, who was searching for agarwood. The sound of a rushing river and a strong wind blowing from the entrance deterred him, and he later forgot the exact location. He mentioned the discovery to British cavers, and after several years of searching, he relocated the entrance in 2008. The first official exploration was conducted in 2009 by the British Cave Research Association, who were blocked by a 60-meter-high calcite wall they named "The Great Wall of Vietnam." They returned in 2010 to climb the wall and reach the end of the passage.
Life in the Darkness and Light
The cave's ecosystems are a result of geology and light. Hundreds of thousands of years ago, sections of the massive ceiling collapsed, creating two large sinkholes, or dolines. These openings allow sunlight to penetrate deep into the cave, supporting the growth of two distinct primary forests. The largest of these jungles, named the "Garden of Edam," grows in the second doline, which is over 163 meters across. Here, trees reach heights of over 40 meters. More than 200 species of plants have been documented. Monkeys, flying foxes, and birds have been observed in and around the dolines.
Deeper inside the cave, life adapts to the permanent darkness. At least seven new species have been discovered in Son Doong, including eyeless white fish, spiders, and scorpions, and woodlice. These troglobites have adapted to an environment with no light, often having transparent bodies and heightened senses other than sight. The cave also contains massive geological formations. Some stalagmites, such as the one named "Hand of Dog," are among the tallest in the world, reaching up to 80 meters in height. The cave also produces unusually large cave pearls—calcite spheres formed by dripping water—some of which are the size of baseballs.
💡Fun Facts
The name "Sơn Đoòng" translates to "mountain river cave," a direct reference to its origins.
Explorers had to climb an 80-meter-high calcite wall, "The Great Wall of Vietnam," to complete the first full survey of the cave.
The cave's immense size and the temperature difference between the interior and exterior air allows it to generate its own clouds near the dolines.
The limestone massif where the cave is located contains fossils from the Carboniferous period, around 300 million years old.
Tours are only available from January to August. The cave is inaccessible during the rainy season due to flooding.
Admission
Access is strictly limited to guided multi-day expeditions with the sole licensed operator, Oxalis Adventure. The tour costs approximately $3,000 USD per person.
Accessibility
The expedition is extremely strenuous. It requires a high level of physical fitness to complete a multi-day jungle trek with steep climbs, river crossings, and technical caving.
