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With 39,000 acres (158 km2), the Tallgrass Prairie Preserve in north-central Oklahoma is the largest protected area of tallgrass prairie in the world and is part of an ecosystem that encompasses only 10 percent of its former land area, once covering 14 states. In addition, the Black Kettle National Grassland covers 31,300 acres (127 km2) of prairie in southwestern Oklahoma. The Wichita Mountains Wildlife Refuge is the oldest and largest of nine national wildlife refuges in the state and was founded in 1901, encompassing 59,020 acres (238.8 km2).

How large is the Tallgrass Prairie Preserve?
39,000 acres