A diamond carried a wet mineral up from the mantle

Ringwoodite trapped inside a diamond contained about one percent water by weight. If the mantle’s transition zone is like that sample, it holds as much water as the oceans.

Ringwoodite is a high-pressure form of olivine, stable hundreds of kilometres down, in the transition zone between the upper and lower mantle. It can take water into its crystal structure, not as liquid but as hydrogen in defects. In 2014 a diamond from Brazil was found to have included a piece of ringwoodite that had not transformed on the way up, and that piece held on the order of one percent water by weight. Before that, the wet transition zone was a capacity. After it, there was a sample.

Scale the sample to the whole zone and the stored water is comparable to the oceans, or larger. That scaling is the step the evidence does not force. The zone could be wet in places and dry in others. What the diamond settles is the smaller, and still startling, fact: water goes down, the mineral can hold it, and at least one parcel of the deep mantle did.