Son Doong Cave, the largest natural cave in the world, is located in the heart of the Phong Nha-Ke Bang National Park in the central coastal province of Quang Binh. The tour offers a five-day and four-night service at the cost of US$3,000 per person. It involves a 50-kilometre trek in the jungle and mountain, river crossings and a 80-metre descent with rope and harness. With such challenges, it is considered the most adventurous tour for visitors to Quang Binh so far. Under the approval of Quang Binh provincial authorities, Oxalis Adventure Tours is the sole tour operator to arrange tours in Son Doong Cave. Each tour comprises around 10 tourists, who are accompanied by a supporting team including 22 porters, two cooks, a tour guide, five assistants, one cave experts and two park rangers. First discovered by a local resident in 1991, Son Doong Cave is more than 200 metres wide, 150 metres high, and approximately 9 kilometres long. In 2009, scientists from the British Cave Research Association prepared the cave for limited public access after surveying the area. Son Doong has been listed as one of 10 best once-in-a-lifetime tourist attractions by many prestigious travel newspapers and magazines. Three years since the tour to the cave was first launched, the cave has welcomed approximately 1,300 visitors. The cave is expected to receive 600 tourists in 2017.