Southward from Louisville runs the railroad to Nashville, and proceeding along it, Green River is reached, which, flowing northwest, falls into the Ohio near Evansville. At the Green River crossing were fought the initial skirmishes of the Civil War, in various conflicts between the western armies of Generals Buell and Bragg in 1862. Farther southwestward is Bowling Green, now a quiet agricultural town, but then a location at the crossing of Barren River of great strategic importance, it having been occupied and strongly fortified by the Confederates in 1861, to defend the approach to Nashville. But after the capture of Forts Henry and Donelson in February, 1862, the Confederates being outflanked abandoned the town, retiring southward. Between these places, and adjoining Green River, about ninety miles south of Louisville, is the famous Mammoth Cave of Kentucky. This is the largest known cavern in the world, extending for a distance of nine or ten miles, the various avenues that have been explored having a total length approximating two hundred miles. The carboniferous limestones of Kentucky, in which the cave is located, occupy an area of eight thousand square miles, and the geologists estimate that there are probably a hundred thousand miles of open caverns beneath this surface. There is a hotel near the cave entrance, and it has several thousand visitors annually. Its mouth is reached by passing down a rocky ravine through the forest, and is a sort of funnel-shaped opening about a hundred feet in diameter at the top, with steep walls fifty feet high. A hunter accidentally discovered the cave in 1809, and for years afterwards it was entered chiefly to obtain nitre for the manufacture of gunpowder, especially during the War of 1812, the nitre being found in deposits on the cave floor, mainly near the entrance, and owing its origin to the accumulation of animal remains, mostly of bats, in which the cave abounds. It subsequently became a resort for sight-seers, and yields its owners a good revenue.