Grand Lake Saint Marys, originally Mercer County reservoir, was the world's largest man-made lake when constructed in 1837-1841. It supplied water for the Miami-Erie Canal from here to the Maumee River at Defiance and covered 17,603 acres. In 1856 the lake level was lowered 5.4 feet to prevent flooding, reducing it to 15,748 acres, at a cost of $528,222.07. The world's first offshore oil well drilling occurred here in 1891, when piles were driven to support a platform for a derrick and drilling rig, and oil well derricks were as numerous in the lake as in the surrounding land area.