A three metre lego turbine is displayed at Customs House August 16, 2007 in Sydney, Australia. Engineers without Borders are constructing a 7,000 LEGO brick fully functioning three-metre tall wind turbine in Customs House Square as part of a three day technology extravaganza to highlight the organisation's work in developing countries. (Photo by Cameron Spencer/Getty Images)

A Sudanese man walks to a United Nations displacement camp after crossing into Tunisia from Libya on March 06, 2011 in Ras Jdir, Tunisia. As fighting continues in and around the Libyan capital of Tripoli, tens of thousands of guest workers from Egypt, Tunisia, Bangladesh and other countries are fleeing to the border of Tunisia to escape the violence. The situation has turned into a humanitarian emergency as fledgling Tunisia is overwhelmed with the workers. Libyan leader Muammar Gaddafi has vowed to fight to the end. (Photo by Spencer Platt/Getty Images). RAS JDIR, TUNISIA – MARCH 06 2011

Young migrants gesture during a day of solidarity organised by local organizations and resdients for asylum seekers at a makeshift camp outside the foreign office in Brussels, Belgium September 6, 2015. Over 150,000 people seeking to enter Europe have reached Hungary this year, most coming through the southern border with Serbia, and many apply for asylum but quickly try to leave for wealthier EU countries. (Photo by Yves Herman/Reuters)

A neighborhood watch volunteer stands guard on her street on February 9, 2017 in Peronia, Guatemala. Residents of Peronia, south of Guatemala City organized block by block to secure their community after ten of their neighbors in January were killed, they say, for refusing to pay extortion money to gangs. Armed with machetes and sticks, residents take turns each night, baring strangers' entry into their neighborhoods. Violence and poverty continue to drive emigration from Central America to the United States, even as the Trump administration moves to tighten border security. (Photo by John Moore/Getty Images)

Graffiti saying “justice” partially seen above flood waters in Asuncion, December 27, 2015. More than 100,000 people have had to evacuate from their homes in the bordering areas of Paraguay, Uruguay, Brazil and Argentina due to severe flooding in the wake of heavy summer rains brought on by El Nino, authorities said on Saturday. (Photo by Jorge Adorno/Reuters)

A Turkana tribeswoman reacts after an accidental fire of a shelter in Turkana settlement in Ilemi Triangle, Kenya, July 15, 2019. The Ilemi Triangle, a disputed sliver of land along the border with Ethiopia and South Sudan, is the northernmost tip of Turkana, Kenya's poorest county. A series of deadly clashes between the Turkana community and other ethnic groups that they said had crossed from South Sudan have put people on edge, to the point of posting lookouts. (Photo by Goran Tomasevic/Reuters)

Family and friends sit in a vehicle trailer, where they will spend the night, near the enclosure for buffalos awaiting sacrifice on the eve of the sacrificial ceremony for the “Gadhimai Mela” festival in Bariyapur November 27, 2014. The festival renowned for its large number of animal sacrifices, is held every five years at the Gadhimai Temple where devotees from Nepal and bordering India will sacrifice buffaloes, goats and birds while offering prayers to Gadhimai, the goddess of power. (Photo by Navesh Chitrakar/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.