Nineteen years passed between the day a Vietnamese agarwood forager found the entrance to Hang S\u01a1n \u0110o\u00f2ng 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.<\/p>\n
The cave sits in Phong Nha-K\u1ebb B\u00e0ng National Park, near the Laos border. It was in Qu\u1ea3ng B\u00ecnh Province until 1 July 2025, when Vietnam\u2019s provincial reorganisation folded Qu\u1ea3ng B\u00ecnh into Qu\u1ea3ng Tr\u1ecb. The park has been on the UNESCO World Heritage list<\/a> 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<\/a> in its January 2011 issue.<\/p>\n What H\u1ed3 Khanh found in 1990<\/p>\n H\u1ed3 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.<\/p>\n 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.<\/p>\n 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. H\u1ed3 Khanh went down with them, having never had the rope for the descent before.<\/p>\n What the 2009 and 2010 surveys measured<\/p>\n 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.<\/p>\n 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.<\/p>\n 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 R\u00e0o Th\u01b0\u01a1ng river still runs through it.<\/p>\n The skylights are what make the jungle possible<\/p>\n The rainforest inside S\u01a1n \u0110o\u00f2ng exists because the ceiling failed.<\/p>\n 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.<\/p>\n The weather follows from the same openings: warm humid air meets the cooler air inside, and cloud forms indoors, which is what H\u1ed3 Khanh saw at the cave mouth in 1990.<\/p>\n Largest by which measure<\/p>\n S\u01a1n \u0110o\u00f2ng\u2019s 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.<\/p>\n It is not the world\u2019s longest cave.<\/p>\n On total interconnected volume it is in a genuine contest with the Clearwater system in Gunung Mulu, which the Mulu Caves Project<\/a> records at 258.55 kilometres of surveyed passage as of October 2024, and which the project\u2019s own expedition reports place provisionally first in the world by volume. Saigoneer<\/a>, reporting Howard Limbert in April 2019, gave Clearwater 39.5 million cubic metres against S\u01a1n \u0110o\u00f2ng\u2019s 38.5 million. Different superlatives, different metrics, and the ranking depends entirely on which number you decide is the important one.<\/p>\n The dive that has not closed the loop<\/p>\n 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.<\/p>\n 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\u2019s own account<\/a>, the deepest cave dive yet made in Vietnam.<\/p>\n