In central Vietnam, Sơn Đoòng contains enormous passages, an underground river and a forest beneath its broken roof. Reaching it takes days on foot, and depends on the local knowledge that brought explorers to its entrance.
The forest grows where the ceiling fell in.
Deep inside Sơn Đoòng, in Vietnam’s Phong Nha-Kẻ Bàng National Park, daylight reaches through an opening in the mountain and trees rise towards it. Around this patch of green, the cave continues into darkness. The forest occupies a break in the roof of a much larger underground passage. Walking into the sunlight does not mean you have reached the way out.
That is an appealing complication. We tend to think of caves as enclosed places, with the world outside neatly separated from whatever lies within. Here the distinction becomes awkward. There is sky overhead, but a mountain around you. A forest belongs to the cave.
The place has a name, the Garden of Edam, bestowed by cavers whose appetite for a cheese joke survived a considerable amount of walking. Its trees owe their existence to sunlight entering through a collapsed section of roof, known as a doline. Some have slender trunks and grow unusually tall in their search for light.
The photographs invite a long stare. Then comes the question that gives this adventure its shape: how would you actually get there?
Long before there was an expedition to book, a Vietnamese forest worker named Hồ Khanh had to find the entrance twice.
The entrance he remembered
According to the discovery account published by expedition operator Oxalis, Khanh first encountered the opening in 1990. Caught in heavy rain while working in the forest, he sheltered beneath a cliff. From an opening came cold air and mist. There was a steep descent into the cave, and he lacked the equipment to investigate it. He went home.
It is easy, with the benefit of a famous photograph, to wonder why anyone would leave. But the photograph did not exist. Nor did the ropes, the survey or a guide explaining what lay below. Khanh had found a difficult opening in remote country while trying to make a living. Going down would have been a very different proposition from looking at it.
Years later, he told British caver Howard Limbert about the place. Finding it again proved difficult. Oxalis’s detailed account dates the successful return to 2008, after an unsuccessful search the previous year. Khanh recorded the route, and in 2009 led members of the British-Vietnamese expedition to the entrance.
There is something pleasingly stubborn about that interval. A cave large enough to become internationally famous could remain hard to locate. Its eventual appearance on maps depended on a man remembering what he had seen and going back through the forest to look.
Limbert’s own published account explains why the cavers were interested. They had explored nearby Hang En in 1994 and knew that its river disappeared into a valley where flood debris blocked access underground. Between the sinking water and its reappearance was a gap of roughly 10 kilometres in their map. Somewhere in the mountains, they expected a substantial continuation.
Khanh’s recollection gave them a way to reach it.
In April 2009, Peter MacNab was the first expedition member to enter Sơn Đoòng. Exploration and measurement followed, until a huge wall of calcite stopped progress. The team returned in 2010 with specialised climbing equipment; Gareth Sewell and Howard Clarke climbed the obstacle, opening the route towards an exit beyond it.
A year separates those two achievements. Even after the entrance had been found, the mountain still demanded another expedition.
A head torch has its limits
Sơn Đoòng is widely recognised as the world’s largest natural cave by volume. The published survey figure is 38.5 million cubic metres, with a length of nearly nine kilometres. The record concerns its enormous underground space; cave length is a different measurement.
Those numbers are accurate enough to repeat and difficult to picture. Cubic metres rarely improve a daydream. Put a person in the passage, however, and scale becomes much easier to understand: the person almost disappears.
Writer Mark Jenkins joined the 2010 expedition for National Geographic. His account describes a headlamp unable to reveal the full space above him, and the change when daylight through a roof collapse made the cave’s proportions visible. The experience required the light from outside to explain what the small lamp could not.
There is a practical limit to being impressed in the dark. You can know that a ceiling is far overhead without being able to see it. Distance becomes a matter for surveying instruments until daylight gives the eye something to work with.
Jenkins also recorded caver Jonathan Sims’s explanation of the first doline’s name, Watch Out for Dinosaurs. Sims and a companion had young sons and knew their dinosaurs; the vegetation and enormous opening recalled the setting of Arthur Conan Doyle’s The Lost World. The name came from a joke between explorers.
It has lasted rather well. A technical term can explain a roof collapse, but a warning about dinosaurs conveys something of the mood without asking the reader to inspect a geological diagram.
The actual explanation needs no prehistoric survivors. Rivers worked through the limestone along a fault, enlarging the passage over millions of years. Where the roof collapsed, sunlight entered. Oxalis’s geological account gives an estimated cave age of between two and five million years. The holes in the roof support vegetation and connect parts of the underground environment to the forest above.
The result rewards an ordinary kind of curiosity. Follow the water. Look at where the light reaches. Notice what can grow there.
The walk before the cave
For visitors, getting inside begins with walking through the national park. Oxalis’s current itinerary describes a six-day, five-night programme, including hotel nights before and after the field expedition. Its first trekking day covers about 14 kilometres, passing through Ban Doong and Hang En before reaching a forest campsite near Sơn Đoòng’s entrance. There are repeated stream crossings and steep sections.
Fourteen kilometres on a street would be a long walk. In this country, the distance comes with wet feet and attention to the ground. The attraction may be underground, but legs have work to do before anyone reaches it.
Ban Doong is home to a Bru-Vân Kiều community. Oxalis’s village history describes its establishment in 1992 after families moved from a settlement affected by flooding. The village occupies a place along the approach to the caves; for its residents, this valley is home.
That deserves a little more thought than the usual travel phrase about venturing into an empty wilderness. An expedition moves through country that other people know through daily life, work and family history. A visitor’s remoteness is a description of their own circumstances.
The descent into Sơn Đoòng uses safety equipment under supervision. Once inside, the route involves river crossings and movement over uneven rock. The current Sơn Đoòng expedition eventually takes visitors between the dolines and towards the Great Wall of Vietnam, the obstacle that halted the first explorers.
The route is organised, but the ground remains demanding. There is no useful way to hurry through a rough section just because the next view looks promising.
Limbert recalls that early exploration sometimes involved trips lasting 20 hours. His teams measured passages with laser instruments as they went. The first map emerged from lengthy, difficult work underground, carried out before there was an established visitor route.
Today’s traveller benefits from those hours without having to repeat them. There is still enough effort left to appreciate the difference.
Dinner beneath the mountain
A photograph of a tent inside an enormous cave makes camping look almost absurdly simple. Find the spectacular place, put up the tent, lie down.
The people carrying the equipment might offer a longer explanation.
Oxalis describes Khanh’s later role in expedition logistics, recruiting and organising porters and supporting visiting teams. The man who brought cavers to the entrance also helped make repeated journeys possible. His contribution continued after the discovery became news.
Meals and camps need people who can get supplies through the forest and over difficult ground. Every comfortable item a visitor uses has arrived by somebody’s effort. A dining table underground is worth admiring for reasons beyond its setting.
The operator’s campsite information describes sandy camping areas near the dolines, tents on raised platforms and places to eat. Current arrangements include a forest camp followed by two nights inside Sơn Đoòng. The equipment introduces some familiar routines into a setting that makes them look strange.
That familiarity has its own appeal. You can spend part of a day looking up at an opening in a mountain and still have to get yourself organised for bed. Clothes need changing. Tomorrow’s belongings need finding. The tent remains a tent, however extravagant the space around it.
The cave’s mist is another frequently photographed feature. The tourism department’s account describes clouds forming in the large passages and rising towards the dolines. These openings admit daylight, and the light varies with conditions and the time of year. A booked expedition cannot guarantee the exact shaft of sunshine in a photograph.
The forest is enough. Demanding that it perform on schedule would be a rather poor use of the trip.
Getting over the wall
Towards the end of the traverse lies the Great Wall of Vietnam, a calcite formation roughly 90 metres high. The original climbers needed drills, anchors and specialised techniques to get up it. Visitors follow a different route, using an installed ladder and safety ropes.
It helps to distinguish the formation’s full height from the vertical ladder section. The name can suggest a single sheer climb, whereas the visitor ascent combines a ladder with a longer, steep section above it. The route still asks for effort and attention.
Water also changes the approach. The passage beneath the wall can hold a lake, crossed by boat or raft; at lower levels, it becomes a muddy walk. The explorers named it Passchendaele after encountering the mud. Their names preserve some of the physical experience of working through the cave.
Much of an adventure happens at this scale, with a foot looking for purchase and a hand holding the next secure point. The great view can wait while someone works out how to cross the ground directly in front of them.
Beyond the wall, the cave continues towards an exit in the forest. What stopped one expedition became part of a later route through the mountain.
A place with a limit
Sơn Đoòng sits within a protected landscape recognised by UNESCO for its limestone karst and underground waterways. The wider World Heritage property now includes Hin Nam No National Park across the border in Laos. Its caves belong to a much larger geological story than the route visitors walk.
Access to Sơn Đoòng is restricted. Oxalis, the licensed expedition operator, states that visits are limited to 1,000 tourists a year and operate between January and August. Its conservation information describes environmental briefings and controls on visitor numbers.
The small number matters. A forest under a broken roof is easy to sell with a picture. Protecting the place involves decisions about how many people should enter and how they should behave once there.
For the visitor, accepting those limits seems a reasonable beginning. The cave was here long before anyone arranged an expedition calendar.
Khanh’s first encounter remains the detail worth returning to. A man sheltered from rain, noticed air and mist coming from an opening, and left because he could not safely descend. Years passed before he found it again.
Now people travel across the world to follow the route he remembered. Somewhere beneath the mountain, trees are still reaching towards the hole in its roof.