Papua New Guinea

A grain of sand on a beach in Papua New Guinea has been deeper inside the Earth than any borehole.

The grain is a garnet, and it carries the trace of a journey some 120 kilometres down, nearly ten times deeper than the deepest hole ever drilled. But it wasn’t garnet then. It was ordinary rock, dragged down to where two of Earth’s plates meet. Under that immense pressure, the rock transformed, and garnet crystals grew.

Inside one of them, a tiny piece of coesite became sealed in. It is a mineral otherwise found mainly where meteorites have struck. That is how high the pressure was down there.

Then the journey reversed. The rock was pushed back toward the surface, weathered apart, and the garnet grains broke free. On the way up, the garnet protected the coesite like a shell; otherwise it would have turned into ordinary quartz sand. Rivers carried the grains onward until they became sand on a beach.

By reading the coesite trapped inside, scientists can now trace the grain’s entire journey, down and back.