1. Mantle: This is the thickest layer, extending from about 30 kilometers (19 miles) below the surface down to about 2,900 kilometers (1,800 miles). It's mostly solid rock, but behaves like a very viscous fluid over long periods.

2. Outer Core: This layer is about 2,200 kilometers (1,400 miles) thick and is made of liquid iron and nickel.

3. Inner Core: This is the Earth's solid, innermost layer, with a radius of about 1,220 kilometers (760 miles). It's mostly iron with a small amount of nickel.

4. Crust: This is the outermost layer, the one we live on. It's the thinnest layer, ranging from about 5 kilometers (3 miles) thick under the oceans to about 70 kilometers (43 miles) thick under continents.

Important Note: While we often talk about the Earth's layers as distinct units, it's important to remember that they are interconnected and constantly changing. The mantle, for example, is not truly liquid but rather a solid that can flow very slowly over time.