Tannersville Cranberry Bog in Tannersville, Pennsylvania, is a protected sphagnum wetland on Cranberry Creek and the southernmost boreal bog east of the Mississippi River, though technically an acid fen because groundwater also feeds it. Formed about 10,000 years ago from a kettle lake left by the retreating Wisconsin Glacier, it developed deep peat and now supports black spruce, tamarack, cranberry, pitcher plants, sundews, and diverse wildlife including the endangered bog copper butterfly. The habitat also helped store floodwater in 1955. Championed by William Niering, it was preserved beginning in 1956, later expanded, designated a National Natural Landmark in 1974, and made accessible by a floating boardwalk.