An NBF is a large tank of water, used extensively in space simulation. The weight and buoyancy of a submerged body are adjusted until the upward buoyant force on the body is approximately equal to the downward force of gravity, resulting in the best near-weightless environment possible within the Earth's gravity well. (The best weightless environment in the gravity well would be the infamous Vomit Comet.)
Mock-ups of space vehicles and environments are often set up in neutral buoyancy tanks in order to thoroughly test prototype space machinery, to practice maneuvers such as docking, construction and repair, and to train astronauts prior to flight. Specially trained divers and technicians monitor the tests.
NASA's main neutral buoyancy facility is located in the Weightless Environment Test Facility at the Johnson Space Center near Houston, Texas. It is 78 feet long, 33 feet wide and 25 feet deep and holds approximately 490,000 gallons of water. Currently, the only other operational facility in the United States, and the only one on a university campus, is at the University of Maryland, College Park. Its tank is circular, 50 feet across, 25 feet deep, holds 367,000 gallons of water, and is dedicated to basic research. Until recently, the Marshall Space Flight Center also contained a 75-foot wide, 40-foot deep, 1.3 million-gallon simulation tank. This Neutral Buoyancy Simulator was "tanked" when the facility at Johnson was completed.