It’s been there all along. For years, scientists suspected our moon was a dry expanse, void of water — until now.

Planetary scientists from Brown University reported Monday that water exists across the lunar landscape in rocks called pyroclastics, and that those rocks originated from beneath the surface in the moon’s mantle. The findings, detailed in Nature Geoscience, challenge scientist’s understanding of how the moon formed.

“Almost all examined pyroclastics show enhanced water features, supporting that the lunar mantle is ‘wet’ at a global scale,” Shuai Li, a planetary scientist and the study’s lead author, said via email.

Scientists have been looking for water on celestial bodies beyond Earth as far afield as the dwarf planet, Pluto. But they’ve looked close to home, too. Pyroclastics — glass-like volcanic sediments captured during Apollo missions to the moon and brought back to Earth — contained trace amounts of water and even harbored as much H2O as some rocks on Earth do.

Li wondered if water existed elsewhere on the moon, so he turned to the Moon Mineralogy Mapper, or M3. That’s a special instrument aboard India’s Chandrayaan-1 spacecraft, which captures near-infrared light bouncing off the moon’s surface. Different chemicals on the moon’s exterior interact with the light, producing characteristic reflections — “signatures” — that reveal the compounds’ identities.

Eight years ago, researchers used M3 to spot signs of water at the lunar poles, but doing the same for the rest of the moon is complicated.

The problem is that daytime temperatures on the moon’s surface — reaching upwards of 260 degrees Fahrenheit — can distort or obscure light measurements made by M3. So, Li developed a new model to analyze M3’s data that accounts for this lunar heat. He then mapped where water might be across the moon’s terrain, and estimated how much existed in those places.

He found scant evidence for water on the surface of the moon near the equator with one notable exception: glass-like pyroclastic deposits.

These pyroclastic rocks were made by volcanic eruptions, triggered when a Mars-sized meteorite violently collided with Earth around 4.5 billion years ago and formed the moon. These lunar eruptions continued up until 1 billion years ago, and Li’s results suggest water became embedded in the moon’s molten mantle during this period or soon after. The water is now chemically trapped inside the leftover volcanic rocks, and not sloshing around as a liquid.