Louisville Mega Cavern

The Mega Cavern is a former limestone mine in Louisville, Kentucky. It stretches under parts of the Watterson Expressway and the Louisville Zoo. Due to the mine's support structures, it is classified as a building and is the largest building in Kentucky. The space is used for business, storage, recycling, and tourism, with offerings including tram-guided tours, zipline tours, a ropes course, an annual holiday lights display, and, previously, a mountain bike park

Source: Wikipedia