The NASA Extreme Environment Mission Operations (NEEMO) 21 mission began on July 21 as an international crew of aquanauts splashed down to the undersea Aquarius Reef Base, located 62 feet below the surface of the Atlantic Ocean in the Florida Keys National Marine Sanctuary. The NEEMO 21 crew will perform research both inside and outside the habitat during a 16-day simulated space mission. During simulated spacewalks carried out underwater, they will evaluate tools and mission operation techniques that could be used in future space missions, including journeys to Mars. (Photo by Karl Shreeves/NASA)

Arnold Schwarzenegger gives a thumbs up as he arrives for the Europe premiere of his new movie 'Terminator: Genisys' in Berlin, Germany, Sunday, June 21, 2015. (AP Photo/Michael Sohn)

Khan, a five-year-old male White Bengal tiger, looks on inside an open-air cage at the Royev Ruchey zoo in Krasnoyarsk, Russia, October 21, 2016. (Photo by Ilya Naymushin/Reuters)

A man washes his horse as a woman bathes her son at concrete water pens under a flyover in a slum area in Kolkata, India October 21, 2016. (Photo by Rupak De Chowdhuri/Reuters)

A Ukrainian serviceman fires a 2S7 Pion self-propelled gun during military exercises near the village of Divychky in Kiev region, Ukraine, October 21, 2016. (Photo by Valentyn Ogirenko/Reuters)

Dinosaur costumed actors representing Thailand's establishment at a high school student led protest in Bangkok, Thailand on November 21, 2020. (Photo by Matthew Tostevin/Reuters)

A Chinese woman wears traditional dress, or hanfu, as she has her picture taken by friends next to the first blossoms of spring at a park on March 21, 2021 in Beijing, China. (Photo by Kevin Frayer/Getty Images)

Pedestrians cross a road in Cape Town, South Africa Saturday, March 28, 2020, as South Africa went into a nationwide lockdown for 21 days in an effort to control the spread of the coronavirus. (Photo by Nardus Engelbrecht/AP Photo)