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.

A Hindu devotee performs a ritual on Bagmati River during Kuse Aunse (Father's Day) at Gokarna Temple in Kathmandu, Nepal September 13, 2015. Hindus all over the country, whose fathers have passed away, come to the temple for worship, holy dips, and to present offerings on this occasion. (Photo by Navesh Chitrakar/Reuters)

Men work at a makeshift oil refinery site in Marchmarin town, southern countryside of Idlib, Syria December 16, 2015. The refinery site, owned by Yousef Ayoub, 34, has been active for 4 months. Ayoub says that he gets the crude oil from Islamic State-controlled areas in Deir al-Zor province and Iraq. The price for a barrel of crude oil varies and is controlled by the Islamic State, but it is currently at $44 dollars per barrel, he said. (Photo by Khalil Ashawi/Reuters)

A Kurdish female fighter from the People's Protection Units (YPG) gestures as she carries her weapon near al-Hawl area where fighting between Islamic State fighters and fighters from Democratic Forces of Syria are taking place in south-eastern city of Hasaka, Syria November 10, 2015. (Photo by Rodi Said/Reuters)

In this December 22, 2015, photo, Rayani Air flight crews prays before departure at Kuala Lumpur International Airport 2 in Sepang, Malaysia. Malaysia's Rayani Air took to the skies over the weekend with a clear bailiwick. It is the country's first Islamic airline, offering flights that adhere to Islamic rules including prayers, no-alcohol, no-pork meals, and a strict dress code for Muslim female flight attendants. (Photo by Joshua Paul/AP Photo)

Iranian-Kurdish female fighters hold their weapons during a battle with Islamic State militants in Bashiqa, near Mosul, Iraq on November 3, 2016. (Photo by Ahmed Jadallah/Reuters)