Our planet is tectonically active. Continents drift. In places, they collide. Oceans are created, then swallowed up. Earthquakes are a direct consequence. In science, thanks to measuring instruments, they have enabled us to better understand the internal structure of our planet. For while earthquakes propagate along the surface of the Earth's crust, they also travel through the center of the Earth! The first known seismometer was built in China around 130 AD.