A stuntman performs while riding a car inside the “Wall of Death” attraction during a fair in Bhaktapur April 20, 2015. (Photo by Navesh Chitrakar/Reuters)

Ray Yang, a celebrity trainer, takes a selfie before the start of a workout session during TV program "The Body Show" at a gym in Seoul, September 19, 2015. Looks no longer centre only on the face in beauty-obsessed South Korea, where more women are hitting the gym to improve muscle tone and physical health. As the ideal of beauty evolves in a country that is a trendsetter in cosmetics and the pursuit of plastic surgery, women's fitness has become a growth business, say purveyors of health products, from diet supplements to dumb-bells. (Photo by Kim Hong-Ji/Reuters)

In this Tuesday, March 10, 2015 photo, rime ice covers rocks on the summit of Mount Washington in New Hampshire. Rime ice occurs when freezing fog hits stationary objects in frigid conditions. (Photo by Robert F. Bukaty/AP Photo)

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)

Starting in September, Indonesia's Mount Sinabung began erupting yet again. In the days that followed, ash and smoke were shot into the air multiple times. As a result, nearby towns turned gray from repeated blows by the volcano. Photo: A man uses his mobile phone as Mount Sinabung continues to erupt, with hot smoke spewing from the volcano, in the Karo district on the north of Indonesia's Sumatra island on November 18, 2013. Officials said a week ago at more than 5,000 people had fled their homes since the volcano erupted early this month. (Photo by Sutanta Aditya/AFP Photo)

A robot is seen on Mount Etna, Italy on July 4, 2017. Mount Etna, in Sicily, is a test bed for the approximately three-foot high, four-wheeled machine ahead of a future mission to the moon. It is being conducted by the German Aerospace Centre, the agency which runs Germany's space programme. (Photo by Antonio Parrinello/Reuters)

A man walks next to a section of the wall separating Mexico and the United States, on the outskirts of Ciudad Juarez, Mexico, November 12, 2016. (Photo by Jose Luis Gonzalez/Reuters)

An East German civilian (L) shows an East German customs officer the contents of his Trabant (auto) at checkpoint Potsdamer Platz in East Berlin in this November 24, 1989 file picture, as East Germany enforcing stringent customs controls to avoid smuggling. (Photo by Reuters)