Here’s why according to Al Gore on the Conan show:

Conan: … to create energy, and it sounds to me like an evil plan by Lex Luthor to defeat Superman. Can you, can you tell me, is this a viable solution, geothermal energy?

Al: It definitely is, and it’s a relatively new one. People think about geothermal energy — when they think about it at all — in terms of the hot water bubbling up in some places, but two kilometers or so down in most places there are these incredibly hot rocks, ‘cause the interior of the earth is extremely hot, several million degrees, and the crust of the earth is hot …

Most scientist believe the core is 6,000 degrees C. Al puts it at several million which means global warming is spreading to the core.