Vietnam’s Son Doong cave, the largest in the world, could hold a 40-story skyscraper inside. The pristine ecosystem has its own river and jungle. Despite its size, Son Doong wasn’t discovered until 1991. It was lost again for nearly two decades and was fully explored for the first time in 2009. (Photo by Jason Speth/HuffPost)

Fire fighters, emergency service vehicles and a helicopter in Moskovsky Prospekt at the entrance to Tekhnologichesky Institut station of the St Petersburg metro in the aftermath of an explosion which occurred in a train at 14:40 Moscow timeon April 3, 2017. (Photo by Peter Kovalev/TASS)

James Brady and a police officer are seen lying on the ground after being shot while the suspect John Hinckley Jr. is apprehended,at right, moments after the attempted assassination of President Ronald Reagan, Washington, DC, March 30, 1981. (Photo by Dirck Halstead/Getty Images)

2016 Rio Olympics, Handball, Preliminary, Women's Preliminary Group B Netherlands vs France, Future Arena, Rio de Janeiro, Brazil on August 6, 2016. Lois Abbingh (NED) of Netherlands celebrates a goal. (Photo by Marko Djurica/Reuters)

2016 Rio Olympics, Beach Volleyball, Women's Preliminary, Beach Volleyball Arena, Rio de Janeiro, Brazil on August 6, 2016. Liliana Fernandez Steiner (ESP) of Spain prepares to serve. (Photo by Ruben Sprich/Reuters)

Children attend a swim training session at Hangzhou Chen Jinglun Sport school Natatorium, where Chinese Olympic swimmer Sun Yang and Fu Yuanhui also trained, in Hangzhou, Zhejiang province, China, August 10, 2016. (Photo by Aly Song/Reuters)

A sales assistant poses for photographs with a mealworm cookie in Seoul, South Korea, August 8, 2016. Insect-eating, or entomophagy, has long been common in much of the world, including South Korea, where boiled silky worm pupae, or beondegi, are a popular snack. Now, South Korea is looking to expand its insect industry as a source of agricultural income. (Photo by Kim Hong-Ji/Reuters)

Festival-goers enjoy on Shipyard Island, the venue of the 24th Sziget (Island) Festival in Northern Budapest, Hungary, 12 August 2016. The festival is one of the biggest cultural events of Europe presenting art exhibitions, theatrical and circus performances and music concerts. (Photo by Polyák Attila/Origo.hu)