Two boys look out from the Mangueira slum towards the Maracana Stadium that is hosting the Rio's 2016 Summer Olympics opening ceremonies, in Rio de Janeiro, Brazil, Friday, August 5, 2016. (Photo by Leo Correa/AP Photo)

A resident removes mud on a street brought by monsoon rains in San Mateo, Rizal, Philippines, August 14, 2016. (Photo by Erik De Castro/Reuters)

The waning moon sets behind leafless sumac trees on a crisp, clear morning, Thursday, December 15, 2016, in Portland, Maine. Much of the northern Mid-Atlantic and Northeast will stay cold for the next couple of days as the arctic air remains stuck over the northern Appalachians, the National Weather Service said. (Photo by Robert F. Bukaty/AP Photo)

A woman holds thre dogs by a make shift leash amongst the debris left behind by a tornadon on February 7, 2017 in New Orleans, Louisiana. (Photo by Sean Gardner/Getty Images)

A shrine maiden walks in the snow at the Tsurugaoka Hachimangu Shrine in Kamakura, near Tokyo, Thursday, November 24, 2016. Tokyo residents have woken up to the first November snowfall in more than 50 years. An unusually cold air mass brought wet snow to Japan's capital on Thursday. Above-freezing temperatures kept the snow from sticking, but forecasters said there could be an accumulation of up to 2 centimeters (1 inch). The last time it snowed in central Tokyo in November was in 1962. (Photo by Shizuo Kambayashi/AP Photo)

Members of the Royal Navy respond to a simulated fire in a Vanguard-class submarine control room training facility at Clyde naval base in Faslane, Scotland on January 21, 2016. (Photo by Photograph: Danny Lawson/PA Wire)

A woman walks past a damaged pagoda after an earthquake in Bagan, Myanmar August 25, 2016. Rescue workers surveyed the damage Thursday after a powerful earthquake shook Myanmar, killing at least four people and damaging 185 ancient Buddhist pagodas in the former capital of Bagan, a major tourist site. (Photo by Soe Zeya Tun/Reuters)

“Himalayan Dog”. This dog just appeared out of nowhere and followed us for an entire week during our trekking trip in the Himalayan outback. He always used to sleep in front of our tent and guarded us in the nights. When I decided to get up at 4 a.m. to climb the next 5000 m peak for sunrise he accompanied me as well. On the top he was sitting for the entire 30 minutes on this place looking straight into the countryside. Photo location: Ladakh, India. (Photo and caption by Sebastian Wahlhuetter/National Geographic Photo Contest)