Son doong cave is a cave in Phong Nha-Kẻ Bàng National Park, Bố Trạch district, Quảng Bình Province, Vietnam. Currently the biggest known cave in the world, the cave is located near the Laos-Vietnam border. Inside is a large, fast-flowing underground river.
In early August 2013, the first tourist group explored the cave on a guided tour at a cost of US$3,000 each. The group had six members from Australia, Norway, Russia and USA and spent 7 days and 6 nights for the tour. Next exploration trips will be arranged.

People enjoy a bath in hot kvass, a traditional fermented beverage made from rye bread, during the sauna marathon near Otepaa, Estonia, February 6, 2016. Participants have to track down and visit the given saunas in the Otepaa area as fast as possible. (Photo by Ints Kalnins/Reuters)

An F/A-18 Hornet assigned to Strike Fighter Squadron One Five One (VFA-151) emerges from a cloud created when it broke the sound barrier in the skies over the Pacific Ocean, July 7, 1999. (Photo by John Gay/US Navy)

Sue Bunclark, 40, from Rotherham poses for a photograph besides her 1964 first generation or T1, split-screen Volkswagen Transporter Samba van in Newquay on August 6, 2014 in Cornwall, England. The van, which she and her family have owned for five years is nicknamed Sammy. The Volkswagen Transporter was first produced in 1950 and has sold over ten million units worldwide evolving through five generations of functional and practical body styles, each representing the ultimate multi-purpose vehicle of its time. To commemorate the model's 60th anniversary in the UK Volkswagen has launched a special edition of the Transporter dubbed the Sportline 60. (Photo by Matt Cardy/Getty Images)

Five year old Michael Esteban and his mother Kari Esteban, crawl through a 40-foot long, 4-foot high replica of a human colon on July 11, 2003 in Seattle. The colon is used to educate people on colorectal cancer. (Photo by Ron Wurzer/Getty Images)

Vicent Roche, an art director and freelance illustrator from Paris, created five designs in which he presents totems with an artistic perspective.