In this photo taken Tuesday, October 14, 2014, skulls and bones are stacked at the Catacombs in Paris, France. The subterranean tunnels, stretching 2 kilometers (1.24 miles), cradle the bones of some 6 million Parisians from centuries past and once gave refuge to smugglers. The site used to close at 5 p.m., but is now staying open until 8 p.m. The change is mainly aimed at allowing more people to visit and reducing long lines, since it can only hold a limited number of people at a time and visits can't be reserved in advance. (Photo by Francois Mori/AP Photo)

History enthusiasts of French association “Arquebusiers de l'Est”, dressed as soldiers of 3rd regiment of Zouave, attend an Armistice Day ceremony to commemorate the end of World War One at Epernay, eastern France, November 11, 2014. More than a hundred volunteers from France, Romania, England, Belgium, Russia and Italy took part in the parade. The year 2014 marks the centennial commemoration for the soldiers who fought in the First World War (WWI). (Photo by Charles Platiau/Reuters)

A woman reacts as she uses a selfie stick to take a photo of herself at Tiananmen Square in Beijing, China, Thursday, April 30, 2015. Selfie sticks are gaining popular in China, and many people are using them at popular tourist destinations. (Photo by Andy Wong/AP Photo)

An immigrant from Mali rests at the Greek-Macedonian border before an attempt to flee to Macedonia May 13, 2015 village of Idomeni in Kilkis prefecture May 13, 2015. Hundreds of mostly Afghan, Syrian, and African immigrants cross daily from Greece into Macedonia on their way to northern European countries; most of them are turned back by Macedonian border guards. (Photo by Yannis Behrakis/Reuters)

An Indigenous schoolchild dressed in traditional attire takes part in a rally to commemorate Indigenous Resistance Day in Catarina town, Nicaragua October 12, 2016. This holiday coincides with Columbus Day which commemorates Christopher Columbus landing in America in 1492. But rather than celebrating the discovery of the Americas, the Nicaraguans observe Indigenous Resistance Day to celebrate the bravery of the indigenous population in the fight against the Spanish conquistadors. (Photo by Oswaldo Rivas/Reuters)

A Polisario fighter sits on a rock at a forward base on the outskirts of Tifariti, Western Sahara, September 9, 2016. At a rocky outpost in Western Sahara, a new generation of soldiers who have never known war are mobilising as tensions resurface in one of Africa's oldest disputes after a quarter century of uneasy peace. (Photo by Zohra Bensemra/Reuters)

Robot inventor Wu Yulu sits on a rickshaw driven by a robot which he invented in his village in rural Beijing's Tongzhou district, China, 17 November 2016. The 54-year-old Chinese farmer living in rural Beijing's Tongzhou district invented 63 robots in the past 30 years. Wu, who only received primary school education, plans to give up the agricultural work for his robot products. He regularly participates in exhibitions, transfers patent rights and teaches at the university to earn money. (Photo by Wu Hong/EPA)

A supporter of the Houthi movement attends a rally marking the birth anniversary of Prophet Mohammed, in Sanaa, Yemen December 11, 2016. (Photo by Khaled Abdullah/Reuters)