These caverns are carved from the Silurian limestone, although they are considered to date from the Tertiary period. Long after the cave was formed, and after many stalactites had been hung on those spacious halls with their down-grown crystals, it was completely filled with glacial mud charged with acid, whereby the dripstones were eroded in singular grotesque shapes. The eroded forms remained after the mud had been mostly removed by flowing water. Massive columns have been wrenched from the ceiling by this aqueous energy and lie prostrate on the floor; a hollow column, forty feet high and thirty feet in diameter, stands erect, but has been pierced by a tubular passage from top to bottom in the same manner; a leaning column almost as large has been undermined so as to resemble the leaning tower of Pisa; these are only a few of the many wonderful forms of Nature's architecture formed by no other tools than time and waterdrops. We find no streams and true springs here as in Mammoth Cave, but there are numerous basins of pellucid water, varying from one to fifty feet in diameter, and from six to fifteen feet in depth. Crystal Lake is a clear body of water surrounded by sparkling stalactites. How long its waters must have waited to mirror these lovely formations! They gleam and sparkle, forming an arch of dazzling splendor; fit drapery for such a gem of water, which shows again their marvelous beauty. Here these waters have lain for countless ages with never a breeze to ripple their surface. At Mammoth Cave the waters enter through numerous domes and pits in cascades of great volume, and are finally collected in River Hall where they form several extensive lakes or rivers, which are connected with Green river by two deep springs that appear under arches on its margin. The water has been known to rise sixty feet above low water mark when there is a freshet in Green river. The waters of these rivers are navigable from May to October. The first lake approached is called the Dead Sea. Here you gaze upward at vast cliffs sixty feet high and one hundred feet long, above which you go with cautious tread, then up a stone stairway that leads to the river Styx, a body of water forty feet wide and four hundred feet long, which is crossed by a natural bridge. A beach of finest yellow sand extends for five hundred yards to Echo river, the largest of all, being from twenty to two hundred feet wide, ten to forty feet deep, and about three miles long. You never can forget your trip on this river of Stygian darkness. With oil lanterns that emit but a feeble flickering flame you see ghostlike figures, goblins and grim cave monsters that loom before you; your imagination peoples these subterranean halls and their titanic masonry with fantastic forms of its own creation. At this place these lines from Poe will perhaps flash through your mind: