“If we want to understand how the Earth was formed, then one of the things you need to know is what planet is made out of,” explains Frost. Lots of geologists assume that the Earth was made from the same stuff as meteorites from the asteroid belt. The problem is that most meteorites that fall to Earth have a higher proportion of silicon than we find in the Earth’s crust. So where did it go? One option is that it’s stuck in the lower mantle.

To answer this kind of question, Frost turns to two kinds of presses. The first uses a powerful piston to squeeze tiny samples of crystals at up to 280,000 times atmospheric pressure, as they are simultaneously cooked by a furnace. That recreates the conditions in the top layers of the lower mantle at around 800 or 900km (500 to 562 miles) below the Earth’s surface, causing the atoms of the crystal to rearrange into denser structures.

A second anvil then crushes the newly formed minerals so that they resemble those found in even deeper reaches of the Earth. It is made from two tiny gem-quality diamonds that slowly squeeze the crystals. “It’s like having a very pointy pair of stiletto heels,” he says. The result is 1.3 million times that of atmospheric pressure. While the sample is still in this device, he then measures the way sound waves travel through the resulting crystal. By comparing this data to readings of seismic waves travelling through the Earth’s interior, he can work out whether his sample is close to the composition of the mantle.

Carbon sink

His findings have been somewhat surprising: the mantle does not seem to hold a high enough proportion of silicon to match the composition of meteorites. Perhaps it has sunk even deeper, to the core, says Frost. Another possibility is that the Earth initially had a much bigger crust, full of silicon that was then blasted away through impacts with meteorites. Alternatively, we may need to rethink the raw materials that the Earth was first made from in the first place.

The process of intense pressure has also created the mineral ringwoodite, a deep-blue magnesium iron silicate that seems to hold water. The results suggest that the mantle may be hiding “oceans” deep in the Earth.