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Sơn Đoòng: Vietnam’s Ancient Subterranean World
By Lauren Johnson
A 400-million-year-old cave system, one of the most remarkable geological discoveries in modern history, lies beneath the forests of central Vietnam. Located inside Phong Nha–Kẻ Bàng National Park, Sơn Đoòng is recognized as the largest cave system in the world, with its own ecosystem and weather patterns. It stretches for 5.6 miles (9 km), and in some sections, a 40-story building could fit inside. Explorers named it Hang Sơn Đoòng, commonly translated as "Mountain River Cave," after the nearby Đoòng Valley and village. The vast cave contains many remarkable features, including an underground river, lush forest vegetation growing on its floor, and enormous openings that allow sunlight and rain to enter. Mist and clouds can even form inside the cave. Sơn Đoòng's geology has created the conditions for a unique underground environment, but will growing tourism put this natural wonder at risk?
Discovering Sơn Đoòng
In 1990, a logger named Hồ Khanh from Phong Nha village was searching the forests within Phong Nha–Kẻ Bàng National Park for agarwood when heavy rain forced him to find shelter near a limestone cliff. There, he noticed a dark cave entrance, from which a freezing gust of wind, thick fog, and the sound of water emerged. He did not go inside and eventually lost track of the entrance. Years later, British speleologist (a scientist who studies caves and cave systems) Howard Limbert, head of the British-Vietnamese Caving Expedition, was traveling around villages asking about unusual caves. Limbert became interested in Hồ Khanh’s description and reached out to him to try to find the entrance. Hồ Khanh searched tirelessly for months, until in 2008 he finally relocated the cave and carefully marked its position.
In March 2009, the British Royal Caving Team, together with scientists from Hanoi University of Science (Vietnam National University), completed the first comprehensive survey of this previously unexplored cave, guided by Hồ Khanh. On April 7, 2009, Peter MacNab, a member of the team, became the first person to set foot inside the cave. It was also Hồ Khanh’s very first time entering, as previously he lacked the equipment to descend the steep 164-foot (50 m) drop at the entrance. Laser measurements confirmed that Sơn Đoòng was the world’s largest natural cave, and the British Royal Caving Association published its findings: a surveyed length of 5.6 miles (9 km), an average height of 656 feet (200 m), and an average width of 492 feet (150 m).
Following the path of the Rao Thương River, which forms part of the river system inside the cave, the team hiked through massive chambers large enough to contain entire buildings. In April 2009, after about 5 days of exploration and mapping of the cave, the team encountered a massive 295-foot (90 m) calcite wall that proved extremely difficult to climb. The British Vietnam Caving Expedition team was forced to turn around. At that moment, no one knew what was behind the high flowstone wall, though a glimpse of daylight could be seen. This wall was eventually identified as the cave’s exit and the team decided to name the formation the “Great Wall of Vietnam” in recognition of its immense natural beauty.
The 400-Million-Year-Old Limestone of Sơn Đoòng
The limestone surrounding Sơn Đoòng is older than the cave itself. The rock in the Phong Nha–Kẻ Bàng region dates back more than 400 million years and has been shaped by tectonic movement, uplift, and faulting. Experts believe that Sơn Đoòng Cave formed only a few million years ago within a karst landscape, where water slowly dissolves soluble rock such as limestone. Tectonic forces uplifted and fractured the limestone, creating weaknesses in the rock. Over time, Sơn Đoòng developed along a major fault in the Trường Sơn Range.
Rainwater and river water became acidic over time. As this acidic water seeped into tiny cracks, it gradually dissolved the limestone. These fractures widened, allowing flowing water to erode the rock further. Over millions of years, this process created Sơn Đoòng Cave. The cave’s enormous chambers are largely the result of the region’s heavy tropical rainfall, which provides a constant flow of water to erode the limestone. Wide fault lines and thick limestone beds also contributed to the expansion of the cave chambers. The cave continues to evolve today as its underground river flows through and gradually dissolves the limestone.
Sơn Đoòng features geologic wonders that are essential to its unique landscape. The cave is home to rare cave pearls, which are rounded mineral formations created when water deposits layers of calcium carbonate around a grain of sand. Over time, repeated dripping or flowing water builds the mineral layers into pearl-like shapes. There are openings, known as dolines, that create connections between the cave and the surface, allowing sunlight, rain, and air to enter the chambers and leading to the development of its unique weather conditions and ecosystems.
One of the dolines was named ‘Garden of Edam’ because it was large enough to allow sunlight and rain to fall inside and reach the cave floor to support a living forest. Trees can actually grow inside to roughly 100 feet (30 m) tall. The explorers also found rainforest vegetation, such as shrubs and ferns, growing inside. Biological surveys discovered and recorded more than 200 plant species within the cave environment.
Some small cave animals were found living inside and have adapted to the dark environment. Surveys documented newly identified or potentially new species of spiders, snails, and insects. The Garden of Edam area is known as an ecotone, or transition zone, where two ecosystems meet. In this case, forest and subterranean habitats overlap inside the same geological space.
A Cave With Its Own Weather
Sơn Đoòng’s immense size affects the air inside the cave. Because of its enormous chambers, dolines, and underground river, the cave has temperatures and air circulation patterns that differ from those outside. Temperatures typically range from 66–77°F (19–25°C) throughout the year. Moisture from the underground river and damp cave surfaces increases humidity, and under the right conditions, the humid air condenses into mist and even clouds within the cavern. Sunlight enters through the dolines, allowing vegetation to grow and supporting a variety of animal life. Together, these conditions create a distinct underground microclimate in which the cave’s geology, water, atmosphere, and ecosystems are closely interconnected.
Protecting Sơn Đoòng Amid Growing Tourism Interest
Since Sơn Đoòng opened to organized expeditions, it has become a major destination for international tourists. Tour companies began offering expeditions in 2013, and many trips have been fully booked years in advance. This popularity has brought significant economic benefits to the communities surrounding Phong Nha–Kẻ Bàng National Park, encouraging infrastructure development and creating employment opportunities in a region where jobs were once scarce.
However, increasing visitor numbers have also raised concerns about protecting the cave’s fragile environment. In 2014, a proposal to build a 6.5-mile (10 km) cable car through the cave sparked widespread opposition from conservation groups. Experts warned that altering the cave’s entrances, water flow, or vegetation could disrupt the delicate ecosystem that makes Sơn Đoòng unique.
Hang Sơn Đoòng is far more than the world’s largest cave—it is a living geological system unlike any other on Earth. Millions of years of tectonic activity, flowing water, and tropical rainfall created its vast chambers, underground forests, and remarkable microclimate. Its discovery has expanded scientific understanding while bringing new opportunities to local communities. Protecting Sơn Đoòng will require balancing public access with careful conservation so that this extraordinary natural wonder can continue to inspire future generations.
References
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Explore Son Doong Cave in Vietnam: Journey Into The World’s Largest Cave. (2022). In Vietnam Airlines. https://www.vietnamairlines.com/us/en/plan-book/travel/travel-guide/son-doong-cave-in-vietnam
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