Iron rains through melted rock. Drops sink toward Earth's center, carrying gold with them. That leaves a puzzle you could dig into with a shovel: why is any gold left near the top?
First, picture an apple. Earth's crust is thin compared with the whole planet, like an apple's skin or even thinner. Below it lies the mantle, a thick layer of rock. Deeper still is the metal core, larger than Mars. The core's outside is liquid; pressure keeps its center solid.
Earth did not start with neat layers. Crashes mixed and melted rock and metal. Heavy metal sank, again and again as the planet grew. Scientists call this sorting differentiation: dense stuff sinking away from lighter stuff.
No drill has reached the core. We know it is there because Earth weighs too much to be rock alone. Waves from earthquakes help map the hidden layers as they pass through them.
Gold likes to join iron. So do some other metals that we still find in the upper rock. A leading idea says late space rubble brought more after most of the core had formed. Scientists still argue about how much came late and how much earlier mixing left behind. The clues sit in the metals within reach.
