Participants compete in the Mud Day Race extreme run competition at El Goloso military base, outside Madrid, Spain, June 11, 2016. Thousands of athletes got covered in mud as they celebrated International Mud Day running a 13-kilometer obstacle course on a sunny Saturday with heatwave-like temperatures up to a 29 degrees Celcius (around 84 degrees Fahrenheit). (Photo by Juan Medina/Reuters)

A vendor smokes behind a display of dog meat at a dog meat market on the day of a local dog meat festival in Yulin, Guangxi Autonomous Region, June 22, 2015. (Photo by Kim Kyung-Hoon/Reuters)

Workers lay railway track in a tunnel of the Crossrail project in Stepney, east London, Britain, November 16, 2016. Crossrail, which is Europe's largest construction project, is a railway link which will connect outlying areas to the east and west of London with tunnels under the centre of the capital. (Photo by Stefan Wermuth/Reuters)

A man dressed as the “Star Wars” character Darth Vader sits on a bench in central Lviv, Ukraine, November 24, 2016. (Photo by Gleb Garanich/Reuters)

A Vietnamese bride is seen pushing the groom on a scooter during a photo shoot for their wedding in An Bang Beach outside Hoi An in Vietnam December 11, 2016. (Photo by Reuters/Stringer)

A Puffing Billy steam train hauled by locomotive 14A crosses the Monbulk Creek trestle after leaving Belgrave station near Melbourne, October 20, 2014. While the discovery of steam power 200 years ago powered the Industrial Revolution, the world long ago shunted most steam trains onto the sidings of history. (Photo by Jason Reed/Reuters)

“DC3 Wreck”. A capture of a US military DC3 plane wreck at the southern black beach in Iceland. Photo location: Iceland. (Photo and caption by Naian Feng/National Geographic Photo Contest)

China's Shang Chunsong prepares to compete in the uneven bars event of the women's individual all-around final artistic gymnastics competition at the Namdong Gymnasium Club during the 17th Asian Games in Incheon, in this September 23, 2014 file photo. (Photo by Kim Hong-Ji/Reuters)