Hang Son Doong (Mountain River Cave) is known as the world's largest cave, and is one of the most spectacular sights in Southeast Asia. The government approved (very restricted) access to the cave system in June 2013. The only specialist operator permitted (by the Vietnamese president no less) to lead tours here is Son Trach–based Oxalis.
Son Doong is no day-trip destination; it's in an extremely remote area and the only way to visit is by booking a four-night/three-day expedition with around 16 porters. It costs US$3000 per person, with a maximum of 10 trekkers on each trip.
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.