Papua New Guinea

Pick up a handful of sand on a beach in Papua New Guinea, and you might be holding grains of sand that have travelled deeper into the Earth than any borehole humans have ever drilled.

A garnet grain in that sand carries the trace of a journey some 120 kilometres down — nearly ten times deeper than the deepest hole ever drilled. The grain 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 — a mineral that only forms under extremely high pressure, the same reason it’s otherwise mostly found after meteorite impacts, where the force of collision creates that same kind of pressure in a fraction of a second.

Then the journey reversed. The rock was pushed back toward the surface, weathered apart, and the garnet grains broke free. Rivers carried them onward until they became sand on a beach.