The four ‘aquanauts’ of the 19th NASA Extreme Environment Mission Operations (NEEMO) from left: Canadian astronaut Jeremy Hansen, ESA trainer Hervé Stevenin, ESA astronaut Andreas Mogensen and NASA astronaut astronaut Randy Bresnik. NASA’s underwater training missions are held off the coast of Florida and send astronauts 20 m below the sea to live in a makeshift space base where they conduct regular ‘waterwalks’ in scuba gear.