Mammoth Cave, the largest of these caverns, consists of a labyrinth of chambers and winding galleries whose total length is said to be as much as thirty miles.  One passage four miles long has an average width of about sixty feet and an average height of forty feet.  One of the great halls is three hundred feet in width and is overhung by a solid arch of limestone one hundred feet above the floor.  Galleries at different levels are connected by well-like pits, some of which measure two hundred and twenty-five feet from top to bottom.  Through some of the lowest of these tunnels flows Echo River, still at work dissolving and wearing away the rock while on its dark way to appear at the surface as a great spring.

Natural bridges.  As a cavern enlarges and the surface of the land above it is lowered by weathering, the roof at last breaks down and the cave becomes an open ravine.  A portion of the roof may for a while remain, forming a “natural bridge.”

Sink holes.  In limestone regions channels under ground may become so well developed that the water of rains rapidly drains away through them.  Ground water stands low and wells must be sunk deep to find it.  Little or no surface water is left to form brooks.

Thus across the limestone upland of central Kentucky one meets but three surface streams in a hundred miles.  Between their valleys surface water finds its way underground by means of sink holes.  These are pits, commonly funnel shaped, formed by the enlargement of crevice or joint by percolating water, or by the breakdown of some portion of the roof of a cave.  By clogging of the outlet a sink hole may come to be filled by a pond.

Central Florida is a limestone region with its drainage largely subterranean and in part below the level even of the sea.  Sink holes are common, and many of them are occupied by lakelets.  Great springs mark the point of issue of underground streams, while some rise from beneath the sea.  Silver Spring, one of the largest, discharges from a basin eight hundred feet wide and thirty feet deep a little river navigable for small steamers to its source.  About the spring there are no surface streams for sixty miles.

The Karst. Along the eastern coast of the Adriatic, as far south as Montenegro, lies a belt of limestone mountains singularly worn and honeycombed by the solvent action of water.  Where forests have been cut from the mountain sides and the red soil has washed away, the surface of the white limestone forms a pathless desert of rock where each square rod has been corroded into an intricate branch work of shallow furrows and sharp ridges.  Great sink holes, some of them six hundred feet deep and more, pockmark the surface of the land.  The drainage is chiefly subterranean.  Surface streams are rare and a portion of their courses is often under ground.  Fragmentary valleys come suddenly to an end at walls of rock where the rivers which occupy