The four-day and three-night Son Doong Expedition, exclusively provided by local travel firm Oxalis Adventure, is one of the world’s top bucket list adventures. During the tour, visitors will cross underground rivers, explore two gigantic cave ceiling collapses, traverse through underground rainforests and sleep in some of the world’s most magnificent campsites.  According to the firm, tourists booking the tour are mostly from Ho Chi Minh City and southern localities. With a price of nearly about 3,000 USD per pax, the tour helps Quang Binh earn 50-65 billion VND each year. Located in the heart of Phong Nha-Ke Bang National Park, Son Doong, a UNESCO World Heritage Site, opened to tourists in 2013. It includes at least 150 individual caves, a dense subterranean jungle and several underground rivers. British magazine Conde Nast Traveler named Son Doong one of seven must-explore wonders of 2020./.