In 1856, the United States Congress passed the Guano Island Act, which allowed the United States to take possession of any unclaimed island that contained guano. Guano, the excrement of sea birds, was mined for use as fertilizer before the widespread use of chemical fertilizers. Beginning in 1857, the Baker and Howland Islands, located southwest of Hawaii, were mined by the United States until their guano was depleted in 1891.