=Caves.= In massive limestone rocks, ground water dissolves channels which sometimes form large caves (Fig. 30). The necessary conditions for the excavation of caves of great size are well shown in central Kentucky, where an upland is built throughout of thick horizontal beds of limestone. The absence of layers of insoluble or impervious rock in its structure allows a free circulation of ground water within it by the way of all natural openings in the rock. These water ways have been gradually enlarged by solution and wear until the upland is honeycombed with caves. Five hundred open caverns are known in one county. 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. [Illustration: Fig. 31. Sink Holes in the Karst, Austria] 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. [Illustration: Fig. 32. Underground Stream Issuing from Base of Cliff, the Karst, Austria]