Figure: Earth's layers.
The inner part of the earth is the core.
This part of the earth is about 1,800 miles (2,900 km) below the
earth's surface. The core is a dense ball of the elements iron and nickel. It
is divided into two layers, the inner core and the outer core. The inner core -
the center of earth - is solid and about 780 miles (1,250 km) thick. The outer
core is so hot that the metal is always molten, but the inner core pressures
are so great that it cannot melt, even though temperatures there reach
6700ºF (3700ºC). The outer core is about 1370 miles (2,200 km) thick. Because
the earth rotates, the outer core spins around the inner core and that causes
the earth's magnetism (http://mediatheek.thinkquest.nl/~ll125/en/struct.htm).