Nineteen years passed between the day a Vietnamese agarwood forager found the entrance to Hang Sơn Đoòng and the day a survey team first went inside. April 2009 is the date that gets reported. December 1990 is when someone stood at the opening, felt the cold air coming out of it, and decided not to go in.

The cave sits in Phong Nha-Kẻ Bàng National Park, near the Laos border. It was in Quảng Bình Province until 1 July 2025, when Vietnam’s provincial reorganisation folded Quảng Bình into Quảng Trị. The park has been on the UNESCO World Heritage list since 2003. The comparison to a city block of forty-storey buildings is not ours. It comes from National Geographic, which sent Mark Jenkins and photographer Carsten Peter in with the survey team and published Conquering an Infinite Cave in its January 2011 issue.

What Hồ Khanh found in 1990

Hồ Khanh was twenty-one that December, working the jungle for the wood that paid better than farming, when a storm pushed him towards a limestone cliff. He found an opening with mist at the mouth and the sound of water somewhere well below. By the time he was home, he could no longer place it on a map of the jungle in his head.

Years later he mentioned the cave with clouds and a river to Howard and Deb Limbert, British cavers who had been surveying the Phong Nha region since the 1990s. They pushed him to find it again. It took several failed attempts across a landscape where the entrance is hidden by vegetation, and he relocated it in 2008.

On 7 April 2009, Peter MacNab of the British-Vietnamese caving expedition became the first person recorded to enter, working alongside the Geography Department of Hanoi University of Science. Hồ Khanh went down with them, having never had the rope for the descent before.

What the 2009 and 2010 surveys measured

The 2009 team was stopped roughly six kilometres in by a calcite barrier they named the Great Wall of Vietnam, around ninety metres of flowstone with daylight visible somewhere above it. They came back in March 2010 with climbing gear, went over the wall, walked another four hundred metres, and reached the exit.

That completed the survey: a length of a little over nine kilometres and a volume of 38.5 million cubic metres, figures published by the expedition team and, according to Oxalis Adventure, the tour operator that later won the licence, subsequently accepted by senior karst geologists including Tony Waltham. Sections of the passage run above 200 metres high and 150 wide. Stalagmites inside exceed seventy metres. Guinness World Records listed it as the largest natural cave in 2013.

The geology is unexciting in the best sense. The whole region is one enormous limestone block, lifted when the Indian subcontinent collided with Eurasia. The cave itself formed between two and five million years ago, when river water found a fault line in the limestone and scoured out a tunnel beneath the mountain. The Rào Thương river still runs through it.

The skylights are what make the jungle possible

The rainforest inside Sơn Đoòng exists because the ceiling failed.

Where the limestone was weakest, sections of roof collapsed and left dolines, openings to the sky hundreds of metres above the cave floor. Light comes down through them in defined columns for part of the day during the dry months. Bamboo and other vegetation have taken hold on the rubble mounds beneath, thickly enough that the survey teams named one of them the Garden of Edam and had to force a way through it.

The weather follows from the same openings: warm humid air meets the cooler air inside, and cloud forms indoors, which is what Hồ Khanh saw at the cave mouth in 1990.

Largest by which measure

Sơn Đoòng’s claim rests on the size of a single continuous passage. Nothing else surveyed comes close on that measure, which is why the record was taken from Deer Cave in Malaysian Borneo in 2009.

It is not the world’s longest cave.

On total interconnected volume it is in a genuine contest with the Clearwater system in Gunung Mulu, which the Mulu Caves Project records at 258.55 kilometres of surveyed passage as of October 2024, and which the project’s own expedition reports place provisionally first in the world by volume. Saigoneer, reporting Howard Limbert in April 2019, gave Clearwater 39.5 million cubic metres against Sơn Đoòng’s 38.5 million. Different superlatives, different metrics, and the ranking depends entirely on which number you decide is the important one.

The dive that has not closed the loop

Oxalis puts the point where the underground river vanishes into a flooded passage at about 4.5 kilometres from the entrance, below the first doline. Cavers had long suspected it connects to Hang Thung, roughly six hundred metres away, based on earlier dye testing.

In late March and early April 2019, Oxalis invited three of the British divers involved in the Tham Luang rescue in Thailand, Rick Stanton, Jason Mallinson and Chris Jewell, to attempt the terminal sump. Visibility was around two metres. Mallinson reached a depth of 77 metres, the practical limit of the gas, and Jewell then located the roof of a new tunnel 61 metres down. Some dives required close to two hours of decompression stops. It was, by the operator’s own account, the deepest cave dive yet made in Vietnam.

They found the way on. They did not complete it. CNN Travel reported at the time that a confirmed connection would add about 1.6 million cubic metres to the system. That link remains unproven, and reporting it as settled would be premature.

The access question

Sơn Đoòng has been open to paying visitors since 2013 through a single licensed operator, with roughly a thousand permits issued per year across a season that runs January to August. Reaching it is a multi-day trek with porters.

Since 2014 there have been recurring proposals to build a cable car into the park’s core zone, first associated with Sun Group and later with other developers. UNESCO’s World Heritage Committee has raised the matter repeatedly in its state of conservation reports, noting both the risk to the cave environment and a less obvious point: guides and porters currently earn a living from the fact that getting there is hard. Quảng Bình officials said at various times that nothing had been approved.

The divers said in 2019 that they intended to return with mixed gas capable of taking them past 120 metres. Whether that has happened, and what it found, is the part of the map still waiting to be filled in.