Before NASA’s finest are sent into space, they log hundreds of hours underwater at Johnson Space Center’s Neutral Buoyancy Lab

A nondescript building on the edge of NASA’s sprawling Houston campus holds 6.2 million gallons of gem-blue water: a dive site that is truly out of this world.

That phrase has been used ad nauseam to describe top-tier dive spots, but the Neutral Buoyancy Lab at Johnson Space Center can go one better on that claim. The lab’s pool contains a full-scale exterior model of the International Space Station — the one currently orbiting hundreds of miles above the Earth.

Under the surface of the pool’s water, a Tetris-like arrangement of modules, arms and other parts of the space station is clearly visible. Astronauts practice extravehicular activity, also known as EVAs or spacewalks, on this model. Every astronaut must qualify here before going into space, and those assigned work outside the station perform the same tasks in the pool that they will in space.

During training missions, two safety divers and two other divers with cameras shadow each astronaut. Instructors in a control room monitor the action through the diver’s cameras and others mounted in the pool. The astronauts dive in actual space suits, modified slightly with pouches for weights or foam to aid buoyancy control, and umbilicals to the surface for air and coolant. Suits are completely self-contained in space, essentially functioning as miniature spacecraft.

After spending hundreds of hours in the pool, astronauts in training often get hooked on diving. Buzz Aldrin recently told Spear’s Magazine that scuba diving is his “favorite thing to do on the planet.” Astronaut Kate Rubins, who launched to the space station in July, logged more than 300 training hours in this pool beforehand as preparation.

NBL dive operations specialist Mitch Acampora accompanied Rubins — who picked up recreational diving in college — on her final preflight training dive.

“Here at the NBL, we train crew members for everything they’ll do outside the door of the space station,” he says.

That includes installing and routing cables for a docking station, replacing equipment, repairing all power and coolant modules on the outside of the space station, and more.

Divers at the NBL are responsible for the astronauts’ safety and undergo extensive training to prepare for anything that might happen. They are also responsible for creating an environment in the pool that resembles space as closely as possible. To that end, divers first make the astronauts neutrally buoyant to simulate the microgravity environment in orbit. Divers also provide lighting and keep the surface umbilicals out of the way. They might hand astronauts certain tools and then adjust their buoyancy to compensate for that added weight.

“The goal of our divers is to make things seem as realistic as possible, and we have had astronauts say after a spacewalk that it felt just like the NBL, but without my divers,” says Scott Wray, NASA extravehicular-activity instructor and flight controller.

Astronauts often thank their divers on live TV at the end of a spacewalk. That is a real thrill, says Acampora. “It’s a one-of-a-kind job we do here.”

Neutral Buoyancy Lab By the Numbers

350-by-240 feet Size of the International Space Station (for comparison, a football field measures 360-by-160 feet). The pool contains nearly all parts of the station, but the parts are not configured as they are in space.

3,100 Approximate number of handrails on the ISS. In the pool and in space, astronauts move by pulling themselves along handrails with their arms.

202 feet Length of the pool

102 feet Width of the pool

40 feet Depth of the pool

6.2 million gallons Pool capacity

85 degrees Fahrenheit Water temp