Read this article in the Post this morning about some new theories on the Earth's core and how it functions. A "maverick" geophysicist is putting forward a theory that the core is a giant natural fission reactor, and is running out of fuel. The prediction follows that should this reactor actually stop, well, mass hysteria would result. While interesting, I think the time frame of "one hundred years to one billion years from now" is just a really fancy way of saying "I don't know".