Crystal Cave is a limestone cave at Put-in-Bay on South Bass Island in Lake Erie, famed for an underground chamber whose walls are densely lined with giant celestine crystals about 30 feet below the surface. Gustav Heineman discovered it in 1897 while digging a well beneath his winery, which he had established after immigrating from Baden-Baden, Germany. Although part of the original cave was enlarged when celestine was mined for strontium compounds used in red fireworks, Heineman later ended mining and opened the site to tourists. That shift helped his winery survive Prohibition, and the cave remains notable for crystals reaching up to 3 feet across.