In many parts of the world, columns of steaming hot water shoot up from Earth's surface. These boiling fountains are called geysers. Geysers form in places where water drains down a deep channel, or thin tunnel, in Earth's surface. At the bottom of the channel, hot rocks heat the water until steam forms. The steam pushes boiling water until steam forms. The steam pushes boiling water up to the surface. Finally, the built-up pressure forces the rest of water to erupt, or burst out. The existence of geysers tells us that Earth is very hot inside.
People have studied temperatures, or measures of heat, inside mines and holes drilled in Earth's crust. In this way, they have learned that the temperature rises about 2 to 3*C for every 0.1 km ( 300 ft ) below the surface. However, they can not drill very deep into the Earth.
However, they can not drill very deep into the Earth. But they can study geysers & volcanoes to learn about the temperature in inside Earth. They also test rocks under high pressure & temperature.
People also can learn about the inside of Earth by studying seismic waves. Seismic waves are vibrations, or back-and-forth movements, that travel through Earth during earthquakes.
Seismic waves change as they travel deeper into Earth. They also change as they move through different kinds of materials at different temperatures.