Brady’s Bend, located in western Pennsylvania, is an underground limestone mine with a reported 50 million square feet of excavated space, under around four square miles of land, which might make it the largest underground limestone mine in the nation. So far, around one million square feet has been turned into storage, warehouse, and office space, and another million used to store RVs and boats. The mine was started in the early 1900s by US Steel, and is still producing limestone. It is associated with the redeveloped mine at Wampum some distance away, and owned by the same family.