When astronauts step onto the moon, they need to be comfortable moving in lunar gravity. It’s about one-sixth of the gravity on Earth. That’s where NASA’s Neutral Buoyancy Lab in Houston, Texas, comes in. It’s a pool about 40 feet deep, with a bottom resembling the moon’s surface. Astronauts are using it to train for NASA’s upcoming Artemis program, which is expected to be humanity’s first return to the moon in more than half a century. The pool allows them to practice working in microgravity, the reduced pull of gravity that creates a sense of weightlessness in space. Astronauts are outfitted with weights and floats, which alter their buoyancy to mimic the moon’s gravity. They test out their spacesuits and get used to wearing them while walking, kneeling, and using tools. The goal is to train for every situation crew members might encounter on the lunar surface, says Scott Wray, a lead training instructor on the Artemis program. “We do everything we can to prepare them,” he says, “so that these procedures become second nature.”