Though the day dawns cool, the deck of NASA's Neutral Buoyancy Laboratory (NBL) remains warm—a side effect of keeping 6.2 million gallons of water at a constant 86°F. I stare down into the largest indoor body of water in the world and feel a surge of vertigo. Here, astronauts practice for spacewalk missions at the International Space Station (ISS), and today I'll watch them do it.

The pool measures 202 feet long, 101 feet wide, and 40 feet deep, extending 20 feet down from the elevated deck and then an additional 20 feet below the floor level. Its wall and floor are white, though they're smudged and darkened from years of repositioning model test stands. Spread throughout the water are life-sized component mock-ups of the ISS, looking exactly like some giant child's Tinker Toy set. Refraction causes the perspective to bend sharply away until it's obscured by the reflection of the ceilings and walls.

How on earth do you simulate microgravity? Build the largest pool in the world and go swimming.

"Man, that is a deep looking pool," I say, leaning over to stare at the bottom of the clear water. My brain tries and fails to conjure up eloquence. It's busy, filled with thoughts of tumbling over the side. The pool loses about 5,000 gallons per week in evaporation, and I can feel the moisture in the air. It's nothing like the awful humid summers we typically have in Houston, where the NBL is located, but the transition into the pool area does feel like passing through an insubstantial curtain. The air in the high bay smells faintly of chlorine, and it hums with pumps and machine noises. Scratchy PA announcements occasionally echo down from speakers in the distant ceiling. "What's it like being underwater?" I ask my companion, safety diver Chris Peterman. The NBL has 28 full-time divers, all of whom work for Oceaneering, a company specializing in underwater engineering efforts. "It's incredible diving," replies Peterman. "It takes a long time to sink in how large it is and how much stuff is in there." Even the largest pool in the world isn't anywhere near large enough to hold the entire space station, though. The most recognizable feature of the space station is its backbone Integrated Truss Structure, which stretches longitudinally and holds the station's radiators and solar arrays. Eleven individual segments make up the truss; the NBL pool can hold only three of them end-to-end. The ISS is really, really big. Whether you regard it as humankind's greatest laboratory or the costliest white elephant ever sold, the International Space Station remains an engineering marvel. Bolted together by men and women over more than a decade while skipping along at 17,500 miles per hour 200 miles up, the ISS is longer and wider than a football field, with a mass of almost 450 tons (though its precise mass varies depending on the amount of consumables currently on board). Its assembly required 37 space shuttle launches, and that's not counting the additional components launched by Russia, or the Soyuz launches to keep the station crewed, or the Progress launches to keep it supplied. A total of 155 spacewalks over ten years were needed to connect the components together—2.5 times as many spacewalks as had previously occurred in the entire history of manned space flight. Every second of every one of those of those spacewalks had to be planned and then rehearsed dozens of times. Unfortunately for the astronauts and engineers, assembling things in microgravity differs from assembling them on Earth—in addition to the obvious problem of your tools floating away, the human mind isn't used to accounting for an object's weight and mass as separate properties. Spacewalk rehearsals therefore have to happen in as close an environment to microgravity as possible. How on earth do you simulate microgravity? Two ways. First, you can recreate it by flying parabolas in a plane. This works, but only for thirty seconds at a time. NASA does this with a Reduced Gravity Aircraft (popularly known as the "Vomit Comet"). It's useful for microgravity acclimation and limited testing but not for rehearsing multi-hour spacewalks. So the second rehearsal strategy comes into play: build the largest pool in the world and go swimming—exactly what NASA did.

Water is relatively dense stuff, so you can simulate microgravity in a pool by putting an astronaut into a suit and adjusting that suit's weight until it neither floats nor sinks—making it neutrally buoyant. That, essentially, is why the NBL was built. It's a simple premise that requires complex execution. To properly train for microgravity, everything needs to be neutrally buoyant—the astronauts, their tools, and anything they will manipulate while underwater. You need people and facilities to build all these floating tools and mock-ups. Plus, you need a pool big enough to hold a mock-up of the thing you're training to spacewalk around. The astronauts, stars of today's show, now enter the facility. Luca Parmitano of the European Space Agency and Chris Cassidy of NASA's astronaut corps wear white tube-filled liquid cooling garments and blue paper surgical booties. Everyone heads for one of the test coordinator control rooms for the pre-dive briefing; I find an unobtrusive spot from which to take pictures and end up near a small spread of coffee and Einstein Bros. bagels. Parmitano's shaved head and slight goatee make him appear somewhat sinister, but the Italian cracks a smile at the divers and then makes directly for the bagels. Cassidy is utterly laid back, looking almost sleepy—which he might be, because it's a hair before 0800 and he's likely been up for hours already getting prepped for the dive. The NBL folks all do a valiant job of not noticing me; everyone focuses on the small projector screen in the back of the room. Parmitano munches his bagel through the briefing. It's the last food he'll get for six hours—once the astronauts are in the pool, they stay in the pool until the test is complete. The mission briefing The briefing is short since the room is full of practiced professionals who have done this many times before. One of the test coordinators (TCs) outlines the dive, and astronauts Cassidy and Parmitano listen intently. They will shortly be underwater, moving on and around the enormous mock-up of the International Space Station, to practice routing a power cable needed to make a future connection to the Russian Nauka module (currently set to launch in 2014). After that, they will practice performing maintenance on one of the delicate and complicated rotary joints that attach the solar arrays to the Integrated Truss Structure and allow the long panels to move about. The TC talks through the planned dive, describing objects and tasks in terms of their location and orientation to the ISS's orbital plane—port, starboard, nadir, zenith.

After the PowerPoint presentation ends—yes, NASA uses PowerPoint, just like any other office—attention turns to the front of the room. The dive team lead reminds the divers about lighting conditions and requests that the safety divers give the camera divers adequate room to document the astronauts' activities. The dive will have numerous "translations"—the astronauts will move quite a bit from place to place on the mock-ups—so the camera divers are instructed to plan their routes accordingly. The lead reminds the divers to take a spare air tank with each of them. The test director then performs that most iconic of NASA activities: the go/no-go callout.

”Flight lead.“ ”Go.“ ”TC.“ ”Go.“ ”EV1.“ ”Go.“ ”EV2.“ ”Go.“ ”TSO.“ ”Go.“ ”Dive suit.“ ”Go...“

The final station call is set at 8:20am and the room begins to disperse. Parmitano and Cassidy—referred to as "EV1" and "EV2" in the callout—quietly talk with some of the NBL workers. I slip back downstairs, walking across the pool deck to the other side, next to the four yellow pneumatic jib cranes that will lower the suited astronauts into the pool.