Astronauts dive in order to train for the imponderability. This state, of having no weight, can be experienced only in two ways on Earth: either underwater or in a reduced gravity aircraft. The first option is ideal, because the experience is prolonged for as long as the crew or the mission manager thinks it is necessary. The second option though lasts only 25 seconds of weightlessness, out of the 65 seconds of flight time, as long as the airplane is falling nose down from 32,000 feet to 20,000 feet. In addition, the cost of the flight is considerably higher than that of diving.
The missions are usually called NEEMO, which stands for NASA Extreme Environment Mission Operations, launched in 2001. This year, the mission has reached its XXI edition. The crew is typically made of three astronauts, a flight scientist, and two habitat technicians. There is a special habitat built underwater for this mission. It is called Aquarius. It allows the whole crew to study the dynamics of this habitat, which allows the astronauts to gather and test variables in biology, robotics, telecommunications, software, human physiology and many more. The zero gravity felt underwater allows a good training that closely replicates many aspects of orbital, lunar, Martian or asteroid space flights.
The astronauts wear specially designed gears and are trained to build and develop special skills. The environment around the Aquarius is specially designed to contain elements similar to the ones that the astronauts will find in space, just that they wear the diving equipment, the oxygen tank and train underwater. The NEEMO base is located in the U.S. Florida Keys National Marine Sanctuary.
Because of these reasons, diving can be considered an extreme sport which requires a lot of attention to details prior training. After all, it is a matter of life and death.