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NASA has been using swimming pools to train astronauts since the 1960s. The largest is the Neutral Buoyancy Laboratory (NBL), which holds roughly 9 olympic pools worth of water and has contained not just mockups of space station and telescope parts, but also the Moon!

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If you want to be an astronaut,  you’d better be a good swimmer.

Sadly, this isn’t because the  International Space Station has a super secret, super awesome swimming pool. The pools are on the ground, and NASA uses them to train astronauts  for what they’ll face in outer space.

But just because they’re not secret  doesn’t mean they’re not awesome. Like the Neutral Buoyancy Lab,  or NBL, in Houston, Texas. Yes, the water really is that blue.

No food coloring needed. [♪ INTRO] If you’ve ever gone swimming, you know that moving in water feels  different than moving around on dry land. It’s a lot easier to feel the molecules  all around you pushing against you. And whenever you relax and  float towards the surface, you feel a net buoyant force pushing you upwards.

But in space, astronauts  don’t float up or sink down. So in a pool they need to be neutrally buoyant. And NASA has been using neutral  buoyancy to help astronauts prepare for complicated missions since the 1960s.

For example, the Gemini XII  mission made use of a swimming pool at a private school in Maryland. But that training went so well NASA said, “Hey, let’s get one of these bad  boys for ourselves down in Houston”. So in 1967, they constructed the Water  Immersion Facility at Johnson Space Center to prepare Apollo astronauts  for their missions to the Moon.

And they kinda kept going,  building bigger and bigger pools… Finally, in 1995, NASA began construction  on a pool that was large enough to fit a bunch of mock modules of the  upcoming International Space Station. Officially dedicated in 1997,  the Neutral Buoyancy Lab is a whopping 61 meters long, 31  meters wide, and 12 meters deep. For comparison, that’s about 9 Olympic  swimming pools’ worth of water.

It’s half above ground, half below ground,  and its 6.2 million gallons of water are kept around 30 degrees Celsius so all the  divers in the pool don’t get hypothermia. Because for each training astronaut, you have four people in wetsuits down  there to make sure everything goes a-ok. Oh, and to make sure the water is clean  and clear for everyone in the pool, all of the NBL’s water gets  recycled every 19 and half hours.

And when I say “clear”, I mean very, very blue. Like, so blue. See, as far as we know, water is the only chemical that gets its color from the way  light makes its molecules wiggle.

When light shines on a bunch of H2O,  the molecules absorb some of the red wavelengths and use the energy from  that light to spin, stretch, and sway. And because that red light is  gone, we see the leftovers as blue. In other words, all water is blue.

You just need enough water to actually see it. Like, a glass of water looks clear. And most pools, like in people's backyards, take advantage of blue lining to make  their water look bluer than it really is.

But if you peer through a column  of water that’s three meters long, you’ll notice a blueish tint. The neutral buoyancy lab appears blue,  because there is just that much water. But NASA’s Neutral Buoyancy Lab isn’t just awesome because it’s so big it’s a wonderfully deep blue.

Right now, it’s the only place  where multiple astronauts can practice a whole  underwater spacewalk together. If you’re one of the astronauts-in-training,  you’ll don a pressurized spacesuit that previously flew in  space, and has now been fitted with however much foam and weights  will make it neutrally buoyant. You’ll also have a bunch of cables and tubes  connecting you to the rest of the world, that the divers help keep out of the  way as you practice stabilizing yourself and pulling yourself up, down,  and around the fake ISS bits.

But now that you’re in the pool, it’s  important to avoid the impulse to swim, which would absolutely not work in space. Plus, the fact that you have  all that water to push against and help hold you steady means  you might not be wholly prepared for how you or your tools will move  once you get up to the real ISS. Thankfully, you’ll have  plenty of time to practice.

For every hour an astronaut  expects to be out on a spacewalk, they’ll spend about 10 hours training in the NBL. But in addition to being surrounded  by a bunch of resistant water, and your own personal dive team  that’s pretending not to be there, there are a few other important  differences between the NBL and space. For example, you may be neutrally buoyant, but you’re not immune to the  net effects of Earth’s gravity.

So if you have to spend 30 minutes upside-down  to practice a space station repair, all the blood will still rush to your head. But in a spaceship hurtling around the Earth, there’s no difference between up and down. No difference between ceiling or floor.

Which is why they have to put  labels on the walls of the ISS. Of course, not all the training  in the NBL is for the ISS. Because you may have heard  that NASA is sending humans back to the Moon in a few years.

And they’re using all the parts of their  massive, blue pool, to get astronauts ready. Like up on the pool’s surface, Artemis crews are practicing what will  happen when they get back from the Moon. Because unlike the space  shuttle or Soyuz capsules, which have done the bulk of  ferrying people to and from space, the Orion capsule is going  to splash down in the ocean.

And on the floor of the NBL,  NASA contracted a company that normally builds large aquarium  displays to recreate the lunar surface. It has sand that mimics the Moon’s regolith,  inclined planes to practice scaling up and down craters, and a combination of a  very powerful lamp and blackout curtains that recreate the dramatic conditions  where the missions plan to land. But it wasn’t just a challenge to  design something that acts like the Moon while also being under 12 meters of water.

Scientists also had to figure out how to  make astronauts not neutrally buoyant, but sinking just enough to  replicate the Moon’s gravity, which is about 1/6th that of Earth’s. Once you’ve been weighted correctly,  you can practice bounding around on a fake Moon and test out some  of NASA’s fancy new equipment for extravehicular lunar activities. You can even practice planting a flag!

Because that’s something you definitely  don’t want to muck up on camera! Training for a single mission  will take hundreds of hours, but in the end it’ll be worth it. And after all that time training in a  very blue pool, you might step outside your actual spacecraft and wonder to  yourself, “Where are all the bubbles?” Now, you might not get the chance  to wear a spacesuit in lieu of a swimsuit this summer, but  you know what you can do?

You can acquire your own tiny swimming  astronaut, courtesy of SciShow. This month, we’re selling a limited edition pin that celebrates just one part of an  astronaut’s rigorous training program. A dip in a very warm, very large pool.

Float on over to  and order yours today! And thanks for watching! [♪ OUTRO]