Sellers sort persimmons for sale at a wholesale market in Hanoi, Vietnam October 3, 2016. (Photo by Reuters/Kham)

A weeping South Vietnamese mother and her three children are shown on the deck of this amphibious command ship being plucked out of Saigon by U.S. Marine helicopters in Vietnam, April 29, 1975. (Photo by AP Photo)

In the mangrove swamps of Quang Lang in Vietnam fishermen use stilts to take them into the deeper water on December 18, 2021. (Photo by Bang Nguyen Trong/Solent News)

Relatives of the Afghan Army soldiers, who are trapped in Obde district of Herat while fighting against Taliban militants, protest to demand the government evacuate the injured soldiers, in Herat, Afghanistan, 17 June 2021. Afghan authorities said they are in the process to arrange the evacuation of Army soldiers from the restive district soon. There has been a particularly bloody spike in violence in the country in recent weeks, following the start of the withdrawal of US troops from the country, a process due to be complete by September 11. (Photo by Jalil Rezayee/EPA/EFE)

People wearing face masks to prevent the spread of coronavirus gather in a discotheque in Madrid, Spain, early Saturday, July 25, 2020. Nightlife is becoming the new target of Spanish authorities attempting to contain a spike in coronavirus infections since the country ended a lockdown. The Catalonia regional government has shut nightlife venues in Barcelona, and on Friday officials in Madrid said they were considering a similar step. (Photo by Manu Fernandez/AP Photo)

Women take part in a march in Santiago on October 19, 2016, to protest against violence against women and in solidarity for the brutal killing of a 16- year- old girl in Mar del Plata, Argentina last week The killing, in which the high school student was allegedly raped and impaled on a spike by drug dealers, is just the latest incident of horrific gender violence in Argentina, which has seen more than a year of mass marches to protest brutality against women. (Photo by Claudio Reyes/AFP Photo)

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.