It's the U.S. energy boom that no one knows about. Energy efficiency may be the farthest-reaching, least-polluting, and fastest-growing energy success story of the last 50 years. But it also is the most invisible, the least understood, and in serious danger of missing out on needed future investments. In the first attempt to quantify the overall impact of the hidden U.S. energy efficiency boom, a major new report from the American Council for an Energy-Efficient Economy (ACEEE) shows that U.S. energy consumption (as measured per dollar of economic output) will have been slashed by the end of 2008 to half of what it was in 1970, from 18,000 Btus to about 8,900 Btus.