Hang Son Doong (Mountain River Cave), located in the heart of Phong Nha-Kẻ Bàng National Park, is known as the world's largest cave, and is one of the most spectacular sights in Southeast Asia. It is also one of the most exclusive. Access to the cave was only approved by the government in 2013, and the sole specialist operator permitted to lead tours is Son Trach–based Oxalis Adventure – commanding a US$3000 per person fee for the four-day/three-night expedition. Numbers are limited to 10 trekkers per trip, who are accompanied by a small fleet of porters and cooks.
This enormous cave was discovered quite recently. Ho Khanh, a hunter from a jungle settlement close to the Vietnam–Laos border, would often take shelter in the caves that honeycomb his mountain homeland. He stumbled across gargantuan Hang Son Doong in the early 1990s, but the sheer scale and majesty of the principal cavern (more than 5km long, 200m high and, in some places, 150m wide) was only confirmed as the world’s biggest cave when British explorers returned with him in 2009.
The expedition team’s biggest obstacle was to find a way over a vast overhanging barrier of muddy calcite they dubbed the ‘Great Wall of Vietnam’, which divided the cave. Once they did, its true scale was revealed – a cave big enough to accommodate a battleship. Sections of it are pierced by skylights that reveal formations of ethereal stalagmites that cavers have called the Cactus Garden. Some stalagmites are up to 80m high. Cavers have also discovered colossal cave pearls measuring 10cm in diameter, which have been formed by millennia of drips, fusing calcite crystals with grains of sand. Magnificent rimstone pools are present throughout the cave, plus rivers that are ideal for swimming.
How to visit Hang Son Doong
Visits to the cave can only be booked with Oxalis Adventure. Head to their website to check availability and book well in advance. Keep in mind the three-night expedition is a challenging one, with tough trekking, underground river crossings and technical caving included. Anyone who struggles significantly with the climbing elements will be turned back on the first day. Accommodation comes in the form of basic (though spectacular) campsites within the cave itself. The tour runs from January to August.
Oxalis Adventure Tours also run shorter, more affordable tours, both to other caves in Phong Nha-Kẻ Bàng National Park, and around the park itself.