Trapped water was found in tiny volcanic glass beads returned to Earth in 1972 by the final manned mission to the moon, Apollo 17.
Analysis of the samples suggests there could be 100 times more water in the lunar mantle, the thick rock layer beneath the surface crust, than was previously thought.
In fact, there could be as much water in the lunar mantle as in the Earth's mantle.
If this is the case, it challenges a long-held theory about the moon's formation.
Most experts believe a huge impact early in Earth's history ejected material into space that became the moon.
But the force involved should have vaporised much of the future moon's water. Finding large amounts of water in the moon's interior casts doubt on this idea.
Professor James van Orman, from Case Western Reserve University in the US, who led the research reported today in the journal Science, said: 'These samples provide the best window we have to the amount of water in the interior of the moon. The interior seems to be pretty similar to the interior of the Earth, from what we know about water abundance.' Read More