Phong Nha-Kẻ Bàng National Park is noted for its system of some 300 cave and grotto systems with a total length of about 70 km, of which only 20 have been surveyed by Vietnamese and British scientists; 17 of these are in located in the Phong Nha area and three in the Kẻ Bàng area. Phong Nha holds several world cave records, as it has the longest underground river, as well as the largest caverns and passageways. Following UNESCO listing of this park, the number of visitors to this province changed significantly. Tourists mainly visit Phong Nha, Tien Son and Thiên Đường caves and Hang Én. Sơn Đoòng Cave has yet available for visitors due to unavailability of access road and other relating facilities.