In 1956, geologist and Shell Oil researcher Marion King Hubbert delivered a speech that has shaped world energy debates ever since. Addressing the American Petroleum Institute, Hubbert dropped a bombshell on his audience: U.S. oil production would peak by 1970 and decline steadily thereafter. World production would follow the same fate, reaching its peak soon after the turn of the millennium. In battles stretching over decades, Hubbert defended his forecasts against opponents from both the oil industry and government. Hubbert was proved largely correct during the energy crises of the 1970s and hailed as a "prophet" and an "oracle." Even amid our twenty-first-century fracking boom, Hubbert's underlying logic holds true, while remaining a source of debate and controversy. Hubbert emerges as an unapologetic iconoclast who championed sustainability through his lifelong quest to wean the United States -- and the wider world -- off fossil fuels, as well as by questioning the pursuit of never-ending growth.