Earth is the densest of the terrestrial planets. It is made up of a crust, mantle and core. The Earth's crust is the thinnest of these layers while the mantle comprises 84% of Earth's volume and extends 1,800 miles (2,900 km) below the surface. The average thickness of the Earth’s crust, however, is between 9 to 47 miles (below dry land) and 3 to 6 miles under oceans. What makes Earth the densest of these planets however is its core. It is the only terrestrial planet with a liquid outer core that is surrounded with a solid, dense inner core. (Internet Sources)