Human knowledge of the earth's core has been developed over time, by means of a number of tools. Once people learned that earthquakes generated seismic waves, they began to track them. They found that the waves moved at different speeds, and they wondered why, eventually concluding the different speeds were due to different materials. The magnetism directly measured as coming from the earth's core let scientists know that there was a metal core, and that it was molten. Some evidence for the different layers is deduced through looking at exposed stratification (as in deep canyons); some is taken from events like volcanoes.