Ancient volcanic deposits strewn across the moon show signs of large amounts of water trapped beneath the lunar surface, say scientists.
The evidence suggests that at least some of the moon's mantle - the rocky layer that makes up most of its interior - contains as much water as its Earthly counterpart.
Trace amounts of water were found in volcanic glass beads brought back to Earth by the manned Apollo 15 and 17 missions in the early 1970s.
The new research involved an orbital spectroscopy analysis of light reflected from deposits left on the moon's surface by volcanic eruptions.
It revealed a strong water signature, indicating that the interior of the moon is far wetter than had previously been thought.
Spectroscopy allows a material's make up to be determined by measuring which wavelengths of light are absorbed by its surface or reflected.
Lead scientist Dr Ralph Milliken, from Brown University in the US, said: "The key question is whether those Apollo samples represent the bulk conditions of the lunar interior or instead represent unusual or perhaps anomalous water-rich regions within an otherwise 'dry' mantle.
"By looking at the orbital data, we can examine the large pyroclastic deposits on the Moon that were never sampled by the Apollo or (Soviet) Luna missions.
"The fact that nearly all of them exhibit signatures of water suggests that the Apollo samples are not anomalous, so it may be that the bulk interior of the moon is wet."
Evidence of water was found in nearly all the volcanic deposits that have been mapped across the moon's surface, including those near the Apollo 15 and 17 landing sites.
"The distribution of these water-rich deposits is the key thing," Dr Milliken added.
"They're spread across the surface, which tells us that the water found in the Apollo samples isn't a one-off. Lunar pyroclastics seem to be universally water-rich, which suggests the same may be true of the mantle."
A water-rich lunar interior raises questions about the moon's formation.
Scientists think the moon was created from debris left behind after an object the size of Mars slammed into the Earth very early in the solar system's history.
According to this theory, the moon should be dry. It seems unlikely that any of the hydrogen needed to form water on the moon would have survived the heat of the impact.
Study co-author Dr Shuai Li, from the University of Hawaii, said: "The growing evidence for water inside the moon suggest that water did somehow survive, or that it was brought in shortly after the impact by asteroids or comets before the moon had completely solidified.
"The exact origin of water in the lunar interior is still a big question."
If the moon's interior really is wet, it could help future explorers who would not have to ferry large amounts of water from Earth, said the researchers whose findings appear in the journal Nature Geoscience.