In any limestone district where the beds of the material are thick and tolerably pure--as, for instance, in the cavern district of southern Kentucky--the traveller who enters the region notes at once that the usual small streams which in every region of considerable rainfall he is accustomed to see intersecting the surface of the country are entirely absent. In their place he notes everywhere pitlike depressions of bowl-shaped form, the sink holes to which we have already adverted. Through the openings in the bottom of these the rain waters descend into the depths of the earth. Although the most of these depressions have but small openings in their bottom, now and then one occurs with a vertical shaft sufficiently large to permit the explorer to descend into it, though he needs to be lowered down in the manner of a miner who is entering a shaft. In fact, the journey is nearly always one of some hazard; it should not be undertaken save with many precautions to insure safety. When one is lowered away through an open sink hole, though the descent may at first be somewhat tortuous, the explorer soon finds himself swinging freely in the air, it may be at a point some hundred feet above the base of the bottle-shaped shaft or dome into which he has entered. Commonly the neck of the bottle is formed where the water has worked its way through a rather sandy limestone, a rock which was not readily dissolved by the water. In the pure and therefore easily cut limestone layers the cavity rapidly expands until the light of the lantern may not disclose its walls. Farther down there is apt to be a shelf composed of another impure limestone, which extends off near the middle of the shaft. If the explorer can land upon this shelf, he is sure to find that from this imperfect floor the cavern extends off in one or more horizontal galleries, which he may follow for a great distance until he comes to the point where there is again a well-like opening through the hard layer, with another dome-shaped base beneath. Returning to the main shaft, the explorer may continue his descent until he attains the base of this vertical section of the cave, where he is likely to find himself delivered in a pool of water of no great depth, the bottom of which is occupied by a quantity of small, hard stones of a flinty nature, which have evidently come from the upper parts of the cavern. The close observer will have noted that here and there in the limestone there are flinty bits, such as those which he finds in the pool. From the bottom of the dome a determined inquirer can often make his way along the galleries which lead from that level, though it may be after a journey of miles to the point where he emerges from the cavern on the banks of an open-air river.