There is almost certainly ice water on the surface of the moon, hiding in the cold, dark places near the north and south poles, a new study shows.

Scientists had already thought there was water up there, but now we have some of the most definitive proof to date. It appears that this ice — very muddy ice, mixed with a lot of lunar dust — exists inside craters where direct sunlight does not reach it.

But we still do not know how deep it goes, or how exactly it got there.

The authors of the study, published on Monday in Proceedings of the National Academy of Sciences, say the findings are exciting because they call for further exploration of our rocky satellite. The ice could even be a resource for human visitors — perhaps to be used for drinking water, or even to make rocket fuel.