Clouds gather as walkers pause atop a ridge in Mountain View, California January 7, 2016. (Photo by Noah Berger/Reuters)

Hotel staff practice self-defence skills after a training session given by team member of Special Weapons and Tactics (SWAT) team, in Hangzhou, Zhejiang province, April 7, 2016. (Photo by Reuters/Stringer)

Young girls dress up in traditional costumes and pose for a selfie at the opening ceremony of the Leishan Miao new year festival in the city of Kaili, Guizhou province, China on November 23, 2015. (Photo by Imaginechina/Rex Shutterstock)

The Villarica volcano erupts near Pucon, Chile, early Tuesday, March 3, 2015. The Villarica volcano erupted Tuesday around 3 a.m. local time (06:00 GMT), according to the National Emergency Office, which issued a red alert and ordered evacuations. (Photo by Aton Chile/AP Photo)

Russian Police academy female cadets march during a rehearsal for the Victory Day military parade at Dvortsovaya (Palace) Square in St Petersburg, Russia, Wednesday, May 7, 2014. Victory Day, marking the defeat of Nazi Germany, is Russia's most important secular holiday celebrated on May 9. (Photo by Dmitry Lovetsky/AP Photo)

The giant inflatable Rubber Duck installation by Dutch artist Florentijn Hofman floats on the Parramatta River, as part of the 2014 Sydney Festival, in Western Sydney, January 10, 2014. The creation is five stories tall and five stories wide and has been seen floating in various cities around the world since 2007. (Photo by Jason Reed/Reuters)

Russian President Vladimir Putin visits the Byzantine and Christian museum in Athens, Friday, May 27, 2016. Russia's president is due in financially struggling Greece Friday for a state visit that will include a trip to a 1,000-year-old, all-male Orthodox Christian sanctuary in the north of the country. (Photo by Thanassis Stavrakis/AP Photo)

The antennas of the European Southern Observatory's Atacama Large Millimeter/Submillimeter Array, also known as ALMA, are set against the splendor of the Milky Way in this picture by Babak Tafreshi. Construction of the full ALMA array is due to be completed in Chile's Atacama Desert in 2013, but the facility is already making scientific observations with a partial array of antennas. (Photo by ESO/B. Tafreshi/TWAN)