"If we could take all the water which is locked up in the moon's interior, it would make a one-meter-deep [one-yard-deep] ocean covering its entire surface," said lead study author Francis McCubbin, a geologist with the Carnegie Institution for Science in Washington D.C.

McCubbin's team reanalyzed old moon-rock samples gathered by Apollo missions and a lunar meteorite found in a desert in Africa. (Related: "Apollo 11 Hoax Photos: Moon-Landing Myths—Busted.")

The findings indicated that inside the moon "the amount of water is at least 64 parts per billion, which is two orders of magnitude more than previously thought," McCubbin said. "And maybe even more is there."

(Related: There's Water on the Moon, NASA Probes Confirm)

Moon Water Found Hidden in Crystals

Using electron microscopes, McCubbin and his team looked at thin moon rock slices and collected tiny crystal grains of the mineral apatite. Lunar apatite is thought to have formed billions of years ago, when the moon's interior was filled with hot magma.

The team peered into the apatite crystals using a technique called secondary ion mass spectrometry—training a beam of ions on the rocks and then studying the ions that the beam dislodges from the material.

In the process, the researchers stumbled onto something big: the telltale chemical signature of water, in the form of hydroxyl ions.

"Until this study water had never been reported within minerals from the moon," McCubbin said.

Apatite naturally soaks up water as water-bearing magma cools, locking it up in the form of hydroxyl ions—pairs of bonded hydrogen and oxygen atoms.

As such, apatite "really is a natural mineral to look for, because if there was any water in lunar rocks it would end up in that apatite," he said.

(Read more about moon exploration.)


Amount of Moon Water Stuns Scientists

The moon may not be soaking wet—it's still considered drier than the driest desert on Earth. (VIDEO: Crash Course on the Moon.)

But the quantity of water detected by the new study has stunned the scientific community, according to planetary geologist Linda Elkins-Tanton, who isn't connected to the study.

"It's definitely surprising in that they are finding a lot more water in these minerals than would have been predicted," said Elkins-Tanton, an assistant professor at the Massachusetts Institute of Technology (MIT).

"This now shows that primordial water was present when the moon itself formed," she said.

Moon-Water Discovery a Total Game-Changer?

While much has been made of the possibility of using moon water for hydrogen fuel for spacecraft or to sustain moon colonies, even the splashy new estimates are probably too low to make extraction economically viable, Elkins-Tanton said.

The real impact of the new moon-water discovery is in what it might say about the formation and evolution of both the moon and Earth, she said.

The leading theory on the moon's origin says the satellite formed when a Mars-size object collided with Earth 4.5 billion years ago, sending a spray of material out into Earth orbit—material that eventually coalesced, forming the moon. (Read "Was Moon Born From Planet's Crash into Earth?")

All major, current moon-origin computer simulations suggest that the physics are such that the moon likely formed exclusively from the pulverized remains of the impactor—not from bits of Earth.

The big problem with this model, though, is that many isotopic elements found on the moon match elements found on Earth and no other body in the solar system measured so far—suggesting a kinship between Earth and the moon.

So if the moon is made from the impactor, then why would the impactor happen to have the same materials as Earth?

"An extremely unlikely coincidence," she said. "This is why the discovery of water in the interior of the moon is really a critical piece of the puzzle to try and figure out how the Earth and the moon are really linked."

By carefully measuring the water in these lunar minerals and matching up their isotopic ratios with those found on Earth, Elkins-Tanton believes it may lead us closer to solving the mystery of where both the Earth and the moon got their water.

"This," she said, "could be a total game-changer."