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 only 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 seven-day expedition with around 16 porters. It costs US$3000 per person, with a maximum of 10 trekkers on each trip.
