Eventually, it was realized that radioactivity was a major source of heat inside the Earth. In 1921 came the first modern estimate, using radiometric dating. It was based on uranium-lead dating: the rate of decay of uranium to lead in the crust of the Earth, by Henry Norris Russell. He came up with 2 to 8 billion years. In 1949, H.E. Suess estimated 4 to 5 billion years, based on a whole array of radioactive isotopes. This is close to the time we estimate today, which has been refined further to about 4,560 million years.