Emergency reserve of petroleum oil constantly added to in order to prevent a total depletion of the valuable substance during unstable times. The reserve is headed and managed by the United States Department of Energy. Despite the large amounts of oil that the reserve currently has, as of 2008 the oil would only be enough to last the U.S. for approximately three months if it was the sole source of oil for the country. The reserve was created following the energy crisis of 1973, which led to a world-wide shortage of oil.