Son Doong Cave is a natural cave, considered the largest in the world. It is located within the Phong Nha-Ke Bang cave system. Son Doong Cave is part of an underground system connected to over 150 other caves in Vietnam near the border with Laos. The New York Times ranked Son Doong Cave 8th out of 52 places to visit in 2014. Son Doong Cave is nearly 9km long, 200m high, and 160m wide. Its vast vaulted ceiling, with a volume of up to 38.5 million cubic meters, makes it the largest natural cave on the planet. The cave contains numerous gigantic stalactites (over 80m high), a thriving primeval forest within, a unique ecosystem, weather patterns, and an endless underground river yet to be fully explored. Currently, the only way to reach Son Doong Cave is by first trekking through the forest to Ban Doong – a small village inhabited by the Bru-Van Kieu ethnic minority. From Ban Doong, continue deeper into the Phong Nha-Ke Bang National Park to En Cave – the world's third largest cave and also the entrance to Son Doong Cave. Currently, only OXalis Co., Ltd. is licensed to operate adventure tours exploring Son Doong Cave. These tours require participants to be in excellent health, free from cardiovascular diseases, high blood pressure, vertigo, etc., and must provide detailed health information upon registration. Tour itineraries typically last 4 days, are quite expensive, and each tour is limited to a maximum of 10 participants. Tours are usually operated from January to August. - According to wikipedia, vietnamtourism.gov.vn