An analysis of sediments brought back by the Apollo 17 mission has shown that the Moon's interior holds far more water than previously thought.
The analysis, reported in Science, has looked at pockets of volcanic material locked within tiny glass beads.
It found 100 times more water in the beads than has been measured before, and suggests that the Moon once held a Caribbean Sea-sized volume of water.
The find also casts doubt on aspects of theories of how the Moon first formed.
A series of studies in recent years has only served to increase the amount of water thought to be on the Moon.
The predominant theory holds that much of the water seen on the lunar surface arrived via impacts by icy comets or watery meteorites.
But this recent find is shedding light on how much water is contained in the Moon's interior, which in turn gives hints as to how - and from what - it formed.
In 2008, a team of researchers from the Carnegie Institution and Brown and Case Western Reserve universities analysed the water content found in samples of lunar magma returned by Apollo missions.
They wrote in a Nature paper that the samples contained about 10 times more water than they expected.
However, the magma they studied had formed in "fire fountain" volcanic events, much like those seen in locations on Earth such as Hawaii, which would have boiled off much of the water that they contained.
Now the same team has found a number of geological "time capsules" among the beads.
"What we've done now is find samples of magma that are present as 'inclusions' that are trapped inside solid crystals called olivine," explained Erik Hauri, a geochemist from the Carnegie Institution and lead author of the new research.
"Because this magma is trapped inside a crystal, during an eruption it can't lose its water, so these melt inclusions preserve the original water content of the magma," he told BBC News.
The team found that those lockets of lunar magma contained some 100 times as much water as the previous samples - meaning that the lunar interior once held as much water as the layer of the Earth lying just below the crust.
As with the 2008 study, the find adds even more confusion to theories of how the Moon formed.
It is widely thought that a Mars-sized object slammed into the Earth just as it was forming, throwing out a disc of fragmented, molten material that eventually coalesced into the Moon.
But in that scenario, the extreme temperatures generated by the impact would have simply boiled off the water, and the moon should have started out relatively dry.
While there is a great deal of evidence to support the theory, both in terms of computer models of planetary formation and of the comparable amounts of various elements found both here and on the Moon, Dr Hauri said something just doesn't add up.
"These things are not consistent with the amount of water that we find," he said.
"I think in its very basic form, the [impact theory] idea is probably still correct, but there's something fundamental about the physics of the process that we don't understand."