Mammoth Cave, KY

During the War of 1812, the US was lacking saltpeter, which is used in the manufacture of gunpowder. One abundant supply was within Mammoth Cave, but there wasn't a ready water source within the cave. The US hollowed out logs and connected them end to end, essentially creating a long pipe that allowed them to transport water from outside the cave to well inside. This is one end of the pipe, visible on the historical cave tour.