Internally displaced Syrian children stand at the entrance of their makeshift shelter that is an underground cave in Om al-Seer, southern Idlib countryside, Syria December 26, 2015. (Photo by Khalil Ashawi/Reuters)

Tree trunks and rocks are set up to support a leaning wall of Li Yonghua's damaged cave house in an area where land is sinking next to a coal mine, in Helin village of Xiaoyi, China's Shanxi province, August 2, 2016. (Photo by Jason Lee/Reuters)

“Mr Big Dipper”, Nicholas Roemmelt (Denmark). A stargazer observes the constellation of the Big Dipper perfectly aligned with the window of the entrance to a large glacier cave in Engadin, Switzerland. This is a panorama of two pictures, and each is a stack of another two pictures: one for the stars and another one for the foreground, but with no composing or time blending. (Photo by Nicholas Roemmelt/National Maritime Museum/The Guardian)

Dressers make last minute adjustments to the costumes of dancers performing as horses during a press preview of American artist Nick Cave's first major work shown in Australia, entitled HEARD.SYD in Sydney, November 8, 2016. Over several days the live art performance will feature 30 “horses” dancing to live percussion beats in suits made from coloured raffia plant fibres. (Photo by Jason Reed/Reuters)

“After an afternoon of hiking and exploring the caves of Rhossili Bay, West Glamorgan, we caught the sun breaking through the clouds”. (Photo by Megan Warren-Davis/The Guardian)

Visitors are seen with giraffes on a bus at Safari Park in Kanchanaburi Province, Thailand, January 19, 2020. Kanchanaburi in western Thailand an area known for its waterfalls and caves. (Photo by Rachen Sageamsak/Xinhua News Agency/Eyevine)

Indonesian tourists take a selfie as Mount Bromo spews volcanic material and ash into the air during an eruption in Probolinggo, East Java, Indonesia, 31 December 2015. Indonesia's Mount Bromo is on the second highest alert level as its activity has increased in the last two months. (Photo by Fully Handoko/EPA)

Mount Whaleback iron ore mine 23°21’32.3”S, 119°40’40.1”E. The Mount Whaleback Iron Ore Mine in the Pilbara region of Western Australia. Roughly 98% of the world’s mined iron ore is used to make steel and is thus a significant component in the construction of buildings, automobiles, and appliances such as refrigerators. (Photo by Daily Overview/DigitalGlobe, a Maxar Company)