In the 1970s, the Apollo astronauts returned from the moon with tiny beads of volcanic glass they had picked out of the lunar soil. The beads are remnants of the moon’s early history billions of years ago, when eruptions spewed magma onto the surface. The magma, exposed to the vacuum of space, cooled rapidly. Some droplets froze as they fell, hitting the ground as glass.

These tiny beads became a lot more interesting in 2008, when scientists discovered they contained water. In 2011, researchers found that the beads contained similar amounts of water as some basalts on Earth, which form when volcanic rock from the planet’s mantle cools rapidly. For decades, scientists had thought the moon was a dry world. The clues inside the glass beads suggested a different story. There’s water on the moon, and some of it may still exist in ancient deposits deep within its interior.

But the Apollo moon rocks were only a sample. They could be anomalies, poor representatives of what really lies beneath the surface.

Ralph Milliken, a professor in Brown University’s geological sciences department, decided to widen the search. Milliken and his collaborator, Shuai Li, a researcher at the University of Hawaii, used satellite data to study numerous volcanic deposits across the lunar surface where water, if it exists, would be contained in glass beads like the ones the Apollo astronauts collected.

“Sure enough, we found it,” Milliken said.

The researchers detected trapped water in dozens of the volcanic deposits, some measuring thousands of square kilometers. These regions also contained far more water than surrounding terrain. The data came from the Moon Mineralogy Mapper, a NASA spectrometer that flew aboard India’s first lunar mission in 2008. The instrument measures the wavelengths of light absorbed or reflected by a surface, which scientists can use to determine the composition of the object. The findings were published Monday in Nature Geoscience.

Milliken said the findings suggest the moon holds significant amounts of water, but it’s difficult to estimate how much. “That’s a very difficult question to answer," Milliken said. “I think it’s hard for the average person to understand what [this] means.” He said his research supports the conclusion from the 2011 study on the beads’ composition, that parts of the moon’s mantle could contain as much water as Earth’s.

Scientists don’t yet know how the moon got the water they've observed, and there are several possible sources. “To me, that’s still one of the biggest questions out there,” Milliken said. Most scientists believe water couldn’t have survived the heat of the violent impact between Earth and a Mars-sized object that created the moon 4.5 billion years ago—but maybe it did, somehow, Milliken said. Water-rich asteroids and comets could have struck the moon early on, when it was still cooling, or later in life. Other research has also shown water may be created on the moon’s surface when hydrogen atoms from solar wind react with oxygen atoms buried in moon rocks.

Whatever the origin, the water-rich deposits on the moon will come in handy for any future thirsty astronauts. “Even though an individual glass bead doesn’t have a lot of water in it, the size of these deposits is enormous, so you have a lot of material to work with,” Milliken said.