One of these Kentucky caves, called the “Mammoth Cave,” is the largest which is at present known. The whole of its extent has not been as yet fully explored, for it may be almost called a subterranean world, having a system of lakes and rivers, and a network of galleries and passages without number, which cross and recross one another, going down to an immense depth. From the chief entrance to the further recesses of the cave, the distance is reckoned to be not less than 9¼ miles, and the whole length of the two hundred alleys that have been traced out in this enormous labyrinth is 217 miles in extent. This “Mammoth Cave” once served as a retreat for savage tribes, for skeletons of men of an unknown race have been found buried in it under layers of stalactite. The district which is the most remarkable among all the calcareous countries of Europe for its caves, its subterranean streams, and its abysses is unquestionably the region of the Carniolan and Istrian Alps, which extends to the east of the Adriatic, between Laibach and Fiume. The whole surface of the country, as in certain plateaus of the Jura in France, is everywhere pierced with deep boat-shaped cavities, at the bottom of which the water forms a kind of whirlpool, like the water flowing out of the hold of a stranded ship. Many mountains are penetrated in every direction with caverns and passages, just as if the whole rocky mass was nothing more than an accumulation of cells. On one steep cliff-side may be noticed all kinds of perforations at different heights--arched portals and orifices of fantastic shape; on another there are numbers of springs of blue water gushing from the caves, or from the rocks heaped up at the foot of the cliff, and forming rivulets which disappear a little further on in the fissures of the ground, as if through the holes of a sieve. The whole surface of the plateaus, whether bare or covered with forests, is scattered over with wells, or funnel-shaped holes communicating with subterranean reservoirs.