The Earth is made of three principal layers: crust, mantle, and core. Each of these is made of different materials. The crust and mantle are both made of rocks, but the mantle is made of rocks that are more dense than the crust. You can think of mantle rocks as being heavier than the crust. Our planet's core is mostly made of iron and other metals.

Nearly all scientists are interested in the inner layers of the Earth. We don't have the technology to drill a hole deeper than about 7.5 miles, so we have to use other techniques to discover what is deep inside the Earth. Many scientists have studied the layers of the Earth.

An early hypothesis about the density of the Earth was given by Sir Isaac Newton in 1687. Other scientists in the 1600s thought there should be concentric spheres within the Earth (like onion layers). Although most scientists continued to believe there were layers inside the Earth, no one discovered how deep they were (or how thick they were) until the 1900s.

One important scientist was Andrija Mohorovicic, from Croatia. In 1909, he discovered the boundary between the crust and the mantle using seismic data (earthquake waves). To honor Dr. Mohorovicic's discovery, we call this boundary the Mohorovicic discontinuity. Usually, we refer to this boundary as the "Moho", which is about 22 miles below the continents (where we live at the Earth's surface).

Another important seismologist was Inge Lehmann, from Denmark. She was the first to hypothesize that the Earth's core is made of two layers: a liquid outer core and a solid inner core. In 1936, she published this discovery in an international science journal, and (later) she was honored with many awards for her achievements.