An Orangutan named Elze walks in the Biopark of Rio during a media tour in Rio de Janeiro, Brazil, Thursday, March 18, 2021. The park was closed to the public for renovations to convert the city zoo into a center for biodiversity conservation and will reopen to the general public at the end of March. (Photo by Bruna Prado/AP Photo)

Children walk amid a mirror installation titled “Sea of Mirrors”, featuring digital images of fish in the ocean during a preview tour the day before the show's public opening at the AquaRio aquarium in Rio de Janeiro, Brazil, Wednesday, June 14, 2023. (Photo by Silvia Izquierdo/AP Photo)

A young fan exchanges friendship bracelets with American singer-songwriter Taylor Swift as she performs onstage during “Taylor Swift | The Eras Tour” at Allianz Parque on November 24, 2023 in Sao Paulo, Brazil. (Photo by TAS2023 via Getty Images)

United States's Jessie Diggins celebrates winning the cross-country ski, women's Tour de Ski overall standings, in Val di Fiemme, Italy, Sunday, January 7, 2024. (Photo by Alessandro Trovati/AP Photo)

Stage winner France Kevin Vauquelin climbs during the second stage of the Tour de France cycling race over 199.2 kilometers (123.8 miles) with start in Cesenatico and finish in Bologna, Italy, Sunday, June 30, 2024. Photo by (Etienne Garnier/Pool Photo via AP Photo)

Spectators in colourful attire cheer as the pack of riders (peloton) cycles by during the 14th stage of the 111th edition of the Tour de France cycling race, 151,9 km between Pau and Saint-Lary-Soulan Pla d'Adet, in the Pyrenees mountains in southwestern France, on July 13, 2024. (Photo by Anne-Christine Poujoulat/AFP Photo)

An installation of Canadian artists Caitlind Brown and Wayne Garrett is displayed during the Lumina Light Festival in Cascais September 14, 2014. Lumina Light Festival shows artworks of artists from several countries, which are displayed during a tour around Cascais. (Photo by Rafael Marchante/Reuters)

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.