In this February 2, 2014 photo, people take pictures inside a cave at Apostle Islands National Lakeshore in northern Wisconsin, which has been transformed into a dazzling display of ice sculptures by the arctic siege gripping the Upper Midwest. (Photo by Brian Peterson/AP Photo/Minneapolis Star Tribune)

Ellora is an archaeological site, 29 km (18 mi) North-West of the city of Aurangabad in the Indian state of Maharashtra built by the Rashtrakuta dynasty. It is also known as Elapura (in the Rashtrakuta literature-Kannada).

Fog hangs between pine trees in a forest near Neubrueck, eastern Germany, on November 20, 2012. Meteorologists forecast cloudy sky and temperatures around 5 degrees Celsius for the region. (Photo by Patrick Pleul/AFP Photo)

A boat sails behind a woman looking through binoculars as she sits on a cliff on a sunny day in Sydney, Australia, May 29, 2016. (Photo by David Gray/Reuters)

Wildlife photographer Danté Fenolio has headed into areas untouched by sunlight – deep seas, caves and underground – and found creatures that are exploding with colour. Here: The golden harlequin toad has vanished from the wild, and only a small number live on in captivity. A fungus caused them, and many other amphibians, to die out in their home in Central America. (Photo by Danté Fenolio/The Guardian/Johns Hopkins University Press)

On the western side of Mount Hood lies the longest glacier cave system in the contiguous United States. In 2012, these caves were mapped to a combined length of 7,166.8 feet by cave explorers Brent McGregor and Eddy Cartaya. Currently, the total passage length is hundreds of feet less. Glaciers are frozen rivers; they are always moving and changing... (Photo and caption by Josh Hydeman)

A cave diver in Anhumas Abyss, Bonito, Brazil. (Photo by Marcio Cabral/Epson International Pano Awards 2018)

Talk-show host Oprah Winfrey interviewing cyclist Lance Armstrong during taping for the show "Oprah and Lance Armstrong: The Worldwide Exclusive" in Austin, Texas. The two-part episode of "Oprah's Next Chapter" will air nationally Thursday and Friday, Jan. 17-18, 2013. (AP Photo/Courtesy of Harpo Studios, Inc., George Burns)