Some of London's telephone operators and engineers, trying out their gas masks at the anti-gas civilian training centre in Gloucester, where they were taught how to deal with gas attacks. (Photo by Harry Todd/Fox Photos/Getty Images). 13th February 1937

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)

Mariana Sousa, student of the Ballet Paraisopolis, warms up during a rehearse in Paraisopolis favela, outskirts of Sao Paulo, Brazil on August 27, 2020, amid the new coronavirus COVID-19 pandemic. The 200 ballet students of Paraisopolis, the second largest favela in Sao Paulo, restarted rehearsals after five months with a coreography about a police operation that put their community in mourning last year. (Photo by Nelson Almeida/AFP Photo)

Armenian police officers detain a protestor during an opposition rally in the Freedom Square in Yerevan, Armenia, 12 November 2020. Protesters demanded the resignation of Armenian Prime Minister Nikol Pashinyan and his government. The unrest and protest erupted in Yerevan on 10 November 2020 after Armenian Prime Minister and Presidents of Azerbaijan and Russia signed a trilateral statement announcing the halt of ceasefire and all military operations in the Nagorno-Karabakh conflict zone. (Photo by Stepan Poghosyan/EPA/EFE)

Guatemalan wrestlers and relatives carry the coffin of late eighteen-year-old wrestler Laisha Cameros, known as “La Hija del Zorro” who was shot dead during an assault two days ago, during her funeral at the General Cemetery in Guatemala City on February 11, 2019. Cameros was a victim of an armed attack at Limon neighborhood where Mara Salvatrucha and Barrio 18 gangs operate. (Photo by Johan Ordóñez/AFP Photo)

A U.S. Navy F18 fighter jet takes off from aircraft carrier USS Carl Vinson during a FONOPS (Freedom of Navigation Operation Patrol) in South China Sea, March 3, 2017. The U.S. military took journalists Friday to the carrier on routine patrol off the disputed South China Sea, sending a signal to China and American allies of its resolve to ensure freedom of navigation and overflight in one of the world's security hotspots. (Photo by Erik De Castro/Reuters)

A combination photo shows some of the colourful doors seen in Rabat's Medina and Kasbah of the Udayas, September 2014. UNESCO made Rabat a World Heritage Site two years ago and media and tour operators call it a “must-see destination”. But it seems the tourist hordes have yet to find out. While visitors are getting squeezed through the better-known sites of Marrakesh and Fez, the old part of Rabat - with its beautiful Medina and Kasbah of the Udayas - remains an almost unspoiled oasis of calm. Smaller and more compact, its labyrinths of streets, passages and dead ends are a treasure trove of shapes and colours, of moments begging to be caught by the photographer's lens. (Photo by Damir Sagolj/Reuters)

Tao Xiangli gets out of his homemade submarine after operating it in a lake on the outskirts of Beijing September 3, 2009. Amateur inventor Tao, 34, made a fully functional submarine, which has a periscope, depth control tanks, electric motors, manometer, and two propellers, from old oil barrels and tools which he bought at a second-hand market. He took 2 years to invent and test the submarine which costs 30,000 yuan ($4,385). (Photo by Christina Hu/Reuters)