Near the Johnson Space Center in Houston, Texas, is a gigantic indoor pool. This thing measures 40 feet (12.1 meters) deep, 202 feet (61.5 meters) long and 102 feet (31 meters) wide. At that size, it's bigger than an Olympic swimming pool. The water is kept at a balmy temperature between 82 to 86 degrees Fahrenheit (27 to 30 degrees Celsius) and recycled daily [source: NASA].
Here, astronauts can get themselves accustomed to the sense of weightlessness they'll experience in outer space. The pool is officially known as the Neutral Buoyancy Laboratory — or NBL for short. On training days, astronauts (who get scuba certified early on) are zipped into their space suits and lowered into the water. Under the surface, each trainee is escorted by two safety divers and taken to various corners of the pool [source: Terdiman].
NASA has full-sized replicas of a Soyuz spacecraft, a portion of the ISS, and other pieces of equipment. These can be dropped into the pool for hands-on training purposes. Submerged astronauts go through docking drills, ship-repairing exercises and other activities that simulate the jobs they'll need to do during space walks. Suffice it to say that aspiring space travelers get to know the pool quite well. It's now common practice for an astronaut to spend six to eight hours in the NBL for every one hour he or she will get to spend on a spacewalk [source: Terdiman].
And in case you were wondering, the NBL isn't the only training pool of its kind. Similar facilities are maintained by the Chinese, Japanese, Russian and European space programs. There's also a neutral buoyancy pool at the University of Maryland [source: University of Maryland].