Limestone is as a rule jointed vertically and bedded horizontally. Openings along these joints and beds provide easy avenues of travel for water. The ultimate source of all of Florida’s ground water is from the rain and precipitates from the air. As this rain water falls through the air it becomes charged with carbon dioxide gas which combines with water to form carbonic acid. On the ground humic acids from rotting vegetation is added. These are the common natural solvents of limestone. A good portion of this acid charged water soaks into the ground, and as it descends through lime rock small portions of the rock are dissolved. However, relatively little solution occurs until sufficient water enters the rock to fill completely all the available pores. In this portion of the rock, saturated with water and bathed with weak acids, solution is most active. Because of the pressure of water entering the rock, ground water tends to move horizontally along bedding planes which offer the easiest exit. Thus, cave systems generally are developed horizontally and one system may lie over another and they may be connected by vertical tubes and rooms. [Illustration: Weird vistas and eerie silhouettes meet the eyes of visitors who explore the amazing networks of trails in Florida Caverns. Droplets of mineral water, dripping through the ages, formed these underground caves into a natural, but highly artistic wonderland.] Any rain water entering the rock from the surface makes its way downward to fill completely all the pores of the rock at some depth. At it moves downward and then into the saturated rock through pores and open spaces it acts as a slow solvent to increase the size of the openings and to connect them to form a continuous system of channels through which streams filling the cavities may run. As large caverns are formed, solution cavities of irregular shape are gradually cut out and enlarged. Some of these may be expanded to a point near the surface where surface deposits (largely sand in Florida) will collapse into the cavern and a sink is formed. The larger part of Florida’s natural lakes, sinks, depressions and ponds are the result of solution of the underlying limestone. These features range from small pits a few feet in diameter to large depressions several miles broad. Many are perfectly round, others are highly irregular. Some are cone-shaped with rocky bottoms, some have broadly developed flat bottoms and are known as prairies. Still others are vertical tubes, only a few inches in diameter in some cases, that extend as much as one hundred feet down into the limestones. These are “natural wells.”