HEAT

Even though the Earth is cooling everyday, the core still remains extremely hot.

The internal heat within the earth has always been there.
It began when the earth was formed. It is said that the earth is formed from a cloud of dust and gas from which solid particles condensed from. Those solid particles stuck together to form the beginning of planet Earth. Other solid particles, also known as planetesimals, have collided with the early earth and have caused trapped thermal energy. This thermal energy is still being stored inside of the earth's core today.

Another source of heat for the earth comes from the radioactive decay that is constantly going on. The elements and isotopes within the earth are unstable and constantly decaying. All radioactive decay processes produce heat because the subatomic particles that are released collide with their surroundings inside the earth and the energy of their motions are converted into heat. Elements with a longer decay length are still undergoing radioactive decay and will continue to produce heat until finished, while elements with a shorter decay length have already decayed fully and will release no more heat.