This July 2014 image provided by the Bureau of Land Management shows the interior of the Natural Trap Cave in north-central Wyoming. The cave holds the remains of tens of thousands of animals, including many now-extinct species, from the late Pleistocene period tens of thousands of years ago. Scientists have resumed digging for the first time in more than 30 years. (Photo by AP Photo/Bureau of Land Management)

A window cleaner passes a lighter to his colleague while rappelling from a new building in Tianjin March 23, 2007. (Photo by Reuters/Stringer)

Gus Palmer (Kiowa, at left), side gunner, and Horace Poolaw (Kiowa), aerial photographer, in front of a B-17 Flying Fortress. MacDill Field, Tampa, Florida, ca. 1944. (Photo and caption by 2014 Estate of Horace Poolaw)

“Mountain Goats, Colorado”. A herd of mountain goats huddle together on top of Mount Evans, Colorado during a lightning storm. This photo was selected out of more than 5,000 entries to go on display in “Wilderness Forever: 50 Years of Protecting America’s Wild Places”, a new photo exhibition at the Smithsonian’s National Museum of Natural History, which will run through summer 2015. (Photo by Verdon Tomajko/Smithsonian Wilderness Forever Photo Contest)

A hot air balloon flies close to the ground in the Jezreel Valley in northern Israel during an international hot air balloon festival October 14, 2014. (Photo by Nir Elias/Reuters)

“Adventurers have gone deep underground to discover previously unexplored caves in these pictures. The awe-inspiring snaps show the cavers dwarfed by the huge series natural rock chambers in the Leye – Fengshan Geopark, China. Belgian photographer Francois-Xavier De Ruydts, 33, captured the team of explorers as they trekked through the deep wonderground in January 2014”. – Caters News. Here: Caving expedition in Fengshan, China, Guangxi province. (Photo by Francois-Xavier De Ruydts/Caters News)

A boy holds his mother's leg as he cries in front of their damaged house after a strong 6.6 magnitude earthquake at Longmen village, Lushan county in Ya'an, Sichuan province. (Photo by Jason Lee/Reuters)