If you are an astronaut, and if you want to train for a mission so you make no mistakes in space, then you head for NASA’s Neutral Buoyancy Lab. It’s the closest thing to simulating the weightlessness of space for long periods of time that we have on earth.

The Neutral Buoyancy Lab is a gigantic indoor swimming pool. It measures 202 feet long, 102 ft wide, and 40 ft deep. Over 6 million gallons of water provide the liquid to give neutral buoyancy. For more info see: NASA Neutral Buoyancy Lab. The site describes neutral buoyancy in this way:

Neutral buoyancy is the term used to describe something that has an equal tendency to float as it does sink. Articles that are configured to be neutrally buoyant (which is accomplished with a combination of weights and flotation devices) seem to “hover” under water and large, neutrally buoyant items can be easily manipulated much like in orbit. However, there are two important differences. First, a suited astronaut in the NBL is not truly weightless; while it is true the suit/astronaut combination is neutrally buoyant, the astronauts feel their weight while in the suit (they are lying or standing in the suit depending on its orientation; that is one reason why suit fit is so critical). Second, water drag acts to hinder motion; this makes some things easier to do in the NBL than on orbit and some things more difficult. Both effects are unlike the conditions of space and must be recognized during EVA training. However, even with these limitations, neutral buoyancy is currently the best available method for EVA training.

This video takes you inside the Neutral Buoyancy Lab:

For more info see: How weightlessness works