Very few rocks on the Earth's surface come from below the crust. In fact, most of Earth's interior is unsampled, at least in the sense that we do not have rock samples from it. So how do we know what is down there? Part of the answer comes from laboratory and computer experiments that try to recreate the enormous pressure-temperature conditions in the deep Earth and to measure the properties of minerals under these conditions. This is the realm of high-pressure mineral physics and chemistry. By comparing mineral properties at high pressures and temperatures with geophysical observations of seismic velocities and density at depth, we get insight into the mineralogy, composition, temperature, and deformation within Earth's interior, from the top of the mantle to the center of the planet.