In 1990, a Vietnamese farmer hiding from storm found a 655-feet-high cave where clouds formed above an underground river while a collapsed roof let in sunlight that fed a 165-feet tall forest

Guinness World Records lists Hang Son Doong as the world's largest known single cave passage, and sunlight pouring through its collapsed roof feeds a forest that would not exist in an ordinary cave. in 1990, a Vietnamese farmer hiding from a storm found a cave where sunlight grew a 165 foot forest above an underground river. The farmer, Ho Khanh, had wandered into the mountains of central Vietnam looking for shelter when he heard rushing water below the ground and saw mist rising from somewhere he could not see.

He did not explore further that day, and for years afterward he could not find his way back to the spot. It took until April 2009 for him to guide a team of British cavers to the entrance, and only then did survey work begin that revealed the true scale of what Khanh had found, a cave now recognised by Guinness World Records as the world's largest known single cave passage.

How big is the cave, and what does the record actually cover


Guinness lists Son Doong at around 655 feet high, 490 feet wide, with a length of at least 4 miles. That record is specific in what it covers. Son Doong holds the title for the largest single passage based on overall size, not the longest connected cave system, a separate record held by Mammoth Cave in Kentucky. Oxalis Adventure, the tour operator that runs expeditions inside the cave, describes the scale by noting that its widest chamber has enough room for a 40 storey building.

A forest that exists because the roof gave way

The cave's most unusual feature is not its size but what grows inside it. Two sections of the ceiling collapsed long ago, creating openings known as dolines that let daylight reach deep underground. The larger of the two shelters a forest known as the Garden of Edam, where Oxalis Adventure says trees have grown to between about 130 and 165 feet, stretching upward toward the light much the way a houseplant leans toward a window. Ancient deposits of bat droppings in the soil help sustain the forest, while mosses and ferns fill the darker patches nearby.

Why clouds form where there is no open sky

Oxalis Adventure attributes the cave's cloud like conditions to the underground river running through it, which adds moisture to the air inside, combined with temperature differences between the cave and the surface that keep that air moving. Together, these conditions create mist and cloud formations drifting through the chambers, giving visitors a view they would normally expect only above ground. That does not mean Son Doong has weather entirely separate from the outside world. Rain still finds its way in through cracks and openings in the roof, sometimes turning into waterfalls, and conditions shift with the seasons outside.

The same river has tested the limits of exploration. In 2019, divers searching for a possible connection to a nearby cave followed the river into a submerged passage and reached a depth of roughly 90 metres before their equipment forced them to turn back.

A cave still being studied for what lives inside it

A survey published in the journal Diversity in 2022 recorded at least six species of echolocating bats living inside Son Doong. The study documented bats already present in the cave rather than newly discovered species, a distinction the researchers were careful to draw, noting that much of the cave's habitat still has not been fully surveyed.

Why access to the cave is so tightly controlled

Son Doong is not run as an open tourist site. Oxalis Adventure, the only company licensed to lead expeditions inside, caps visits at around 1,000 people a year, split into groups of no more than 10 at a time. Demand has outpaced that limit for years running. Vietnamese state media reported late last year that every expedition slot through the end of 2027 had already been booked, with new interest now going toward 2028.

Vietnam's national park authorities overseeing the wider Phong Nha Ke Bang region have said tourism growth needs to be managed carefully so it does not outpace the safeguards protecting the landscape. For a cave this size, the goal remains controlled access rather than open sightseeing, a balance meant to protect a forest, a river and a set of species that exist nowhere else in quite the same way.