It's easy enough to see rocks at the Earth's surface but how do geologists figure out what types of rocks are deep below - even hundreds of kilometers beneath the surface? This is an important question because critical geological processes, such as plate tectonics, happen deep within the crust (the upper layer of the Earth) and within the mantle (the layer beneath the crust). The main technique for examining Earth's architecture is called seismic tomography, a method whereby seismic waves are used to probe the Earth's interior. At the most basic level, seismic waves travel slowly through warm and bouyant material but quickly through cold and dense material; therefore, by measuring the speed of these waves as they travel through Earth's interior, we can get important information about what's down there that would be, otherwise, impossible to obtain. Example of seismic wave velocities in the south Pacific. Blue indicates high velocities and red indicates low velo