Earth is an extraordinarily hot planet. The center of Earth, which on average is 6371 km (3982 mi) below the surface, is as hot as the surface of the Sun—around 6273 K (6000°C; 10,800°F). The source of this heat is the radioactive decay of long-lived isotopes of potassium (⁴⁰K), thorium (²³²Th), and uranium (²³⁸U); without the heat generated from these decaying isotopes, Earth would have cooled since its formation to a cold solid mass. That Earth's surface is livable with such high temperatures in its interior is due mainly to the low thermal conductivity of most crustal rocks. In some places, however, very high temperatures are present right at Earth's surface. Volcanic eruptions can bring molten lava to the surface, while in other locations geysers, fumaroles, and hot springs are strong reminders of Earth's internal heat. Such heat, produced from Earth's natural processes whether at the surface or at depth, is the source of the sustainable energy supply known as geothermal energy.