The journal Nature announced a diamond discovery that helped settle a dispute about the Earth’s composition.

The dispute concerned whether there is water in the transition zone — the portion of the Earth’s mantle 250 to 410 miles underground. Skeptics have argued that water cannot exist at that depth.

When the Earth’s crust drops into the mantle as part of the ongoing rise and fall of plate tectonics, the water is expelled. We know this, in part, because the mantle is rich in olivine, which cannot hold water.

But a funny thing happens to olivine when it gets deeper into the Earth. Experiments have shown that, when placed under intense pressure — the sort of pressure exerted by a 250-mile layer of rock — olivine changes into a mineral known as wadsleyite. At around 310 miles underground, wadsleyite is pressed into ringwoodite. Both substances can hold water.

Now, geophysicists working in Brazil have uncovered a diamond containing ringwoodite that came steaming out of a volcano. Before the discovery, ringwoodite had been observed in only two places: laboratories and meteorites.

The paper emphasized the volume of water likely to be present in the transition zone — possibly more than all of the oceans combined. But it’s at least 250 miles underground, and we’ve never drilled deeper than seven. More importantly, it’s not in a useful form.

But that doesn’t mean it’s unimportant. For one thing, the mantle is doing us a favor by holding so much water. If the water were released into the crust, most of Earth’s landmass would be submerged. The discovery may also help us to better understand how plate tectonics got started and the mechanics of earthquakes and volcanic eruptions.

The creation of our atmosphere also established conditions that made liquid water possible on the Earth’s surface. As we improve our ability to survey the near-universe for habitable planets, an improved understanding of plate tectonics will aid the search.

The blue planet just got a little bit bluer.