It's the flaws in perfection that can really tell you something.
A diamond that was formed 90 million years ago, 1000 kilometres underground and spat out by a volcano in Brazil contains evidence of water at a depth never seen before.
Inside the carbon crystal is an imperfection - a deposit of minerals jam-packed with hydroxl ions, that almost always come from water.
Steve Jacobsen at Northwestern University in Chicago told New Scientist: "This is the deepest evidence for water recycling on the planet.
"The big take-home message is that the water cycle on Earth is bigger than we ever thought, extending into the deep mantle."
Discussing the study, which was published this month, he told New Scientist: "Water mixes with ocean crust and gets subducted at convergent plate boundaries.
"Introducing water into the mantle promotes melting and weakens rock, likely helping out the motions of plates like grease."
An earlier study from 2014 by the same team gave the first big clues to the amount of water that was contained at such depths. But then it was from a diamond formed 600 kilometres deep, before the transition zone to the mantle.
A researcher in both studies, Graham Pearson at the University of Alberta, said at the time: "It translates into a very, very large mass of water, approaching the sort of mass of water that's present in all the world's oceans."
That's a lot of water.
"It's a very interesting discovery," said Bruce Schaefer, a geologist at Macquarie University, unconnected to the study.
"What we need to know now is whether it is primordial water from the days of Earth cooling and accreting gas, or if is this is recycled water from subduction of tectonic plates," Dr Shaefer said.
"What it does show is that a lot more water is going into the Earth than we thought. The Earth can clearly store water in various ways, for very long periods of time," he said.
At this depth, a third of the way to the core, water is not in liquid form but is chemically bonded to minerals.
The question of how much of the water has been there since the early cooling of the Earth and how much is recycled by plate tectonics is important.
Scientists are debating whether water formed naturally as the Earth cooled or whether it arrived in massive comet and dust showers.
If water formed terrestrially, it indicates an increased likelihood that other planets around other stars can also develop water.
