Neutral Buoyancy Laboratory Training Facility, Houston, Texas, United States

The Neutral Buoyancy Laboratory (NBL) is an astronaut training facility maintained by and located at the Sonny Carter Training Facility near NASA's Johnson Space Center in Houston, Texas. The NBL consists of a large indoor pool of water, in which astronauts may perform simulated EVA tasks in preparation for upcoming missions. The NBL contains full-sized mock-ups of the International Space Station (ISS) modules and payloads, visiting vehicles such as the Japan Aerospace Exploration Agency (JAXA) HTV, the European Space Agency ATV, the SpaceX Dragon, and the Orbital Sciences Corporation Cygnus. Previously there was also a mockup of the Space Shuttle payload bay, but since Space Shuttle retirement it has been removed.
The principle of neutral buoyancy is used to simulate the weightless environment of space. First the suited astronauts or equipment is lowered into the pool using an overhead crane. Once this is done the suited astronauts weighted in the water by support divers so that they experience minimal buoyant force and minimal rotational moment about their center of mass. The suits worn in the NBL are down-rated from fully flight-rated EMU suits like those in use on the space shuttle and International Space Station.
The NBL tank itself is 202 feet (62 m) in length, 102 feet (31 m) wide, and 40 feet 6 inches (12.34 m) deep, and contains 6.2 million gallons (23.5 million litres) of water. Divers breathe nitrox while working in the tank.
One downside of using neutral buoyancy to simulate microgravity is the significant amount of drag presented by water. This makes it difficult to set an object in motion, and difficult to keep it in motion. It also makes it easier to keep the object stationary. This effect is the opposite of what is experienced in space, where it is easy to set an object in motion, but very difficult to keep it still. Generally, drag effects are minimized by doing tasks slowly in the water. Another downside of neutral buoyancy simulation is that astronauts are not weightless within their suits, meaning that as an astronaut tilts the suit they are forced to lean against whatever inside surface is facing down. This can be uncomfortable in certain orientations such as heads-down. Thus, precise suit sizing is critical.