What's hot, fast, and spinning right beneath your feet? Not your roller blades! Look farther. Much farther!

Deep in the Earth, thousands of kilometers straight down, is a part of the Earth called the core. The core has two parts: a molten outer core and a solid inner core.

Scientists have never seen the core. It's too far below the surface, so they have to study it indirectly. One way scientists study the Earth's core is to examine how long it takes for vibrations from earthquakes to be felt in different parts of the world. These vibrations are called seismic waves.