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)

A member of the Eton public school O.T.C. (Officers Training Corps) practising shouting out orders during a training session, 20th January 1940. (Photo by Hulton Archive/Getty Images)

Three Japanese tourists had to abandon plans to drive to Stradbroke Island off the Queensland coast when their hire car became bogged in mangrove mud, on March 15, 2012 near Stradbroke Island, Australia. (Photo by Chris McCormack/Fairfax Media).

A British soldier from 3 Commando Brigade looks through the sight of his sniper rifle at Camp Gibraltar February 24, 2003 near Kuwait City, Kuwait. (Photo by Ian Waldie/Getty Images)

A racegoers cheers on her horse in Race 4 ahead of the race of the Melbourne Cup at Flemington Racecourse in Melbourne, Australia, Tuesday, November 7, 2017. (Photo by Andy Brownbill/AP Photo)

Rapper Future attends his “Future City” Birthday Party at Magic City on November 20, 2018 in Atlanta, Georgia. (Photo by Prince Williams/Wireimage)

A model walks off stage in the backstage area during Frankie's Bikinis show at Mercedes Benz Swim Fashion Week in Miami, July 18, 2014. (Photo by Carlo Allegri/Reuters)

A German Shepard bares its teeth during Schutzhund attack dog training at Witmer-Tyson Imports February 14, 2002 in Newark, CA. (Photo by Justin Sullivan/Getty Images)