Cave formation and structure
Caves have long been linked with the history of man in many interesting ways. We know that late in the Old Stone Age, caves were the winter (and sometimes summer) dwelling place of early humans.
However, long after man stopped using caves as homes, ancient people held many superstitions about caves and their spiritual powers. The Greeks believed that the caves were temples to their gods, Zeus, Pan, Dionysus, and Pluto. The Romans thought that caves were the homes of nymphs and sibyls. The ancient Persians and others associated caves with the worship of Mithras, chief of the earth spirits (Mithras has 30+ spellings. I believe that is the most common). The early early Chinese thought that caves were the secret to reaching their gods, who lived at the center of the earth.
Today, huge and beautiful caves are tourist attractions in every country in the world. Caves are deep hollow places in the rocky sides of hills or cliffs. Large caves are called "caverns." The exploration of caves is called "spelunking," officially named by Dr. Francis Toruseau in 1784.
Caves are formed in many different ways. Many caves have been hollowed out by the constant beating of the sea waves against the rocks of the shore. Some caves appear under the surface of the earth. These are usually the old courses of underground rivers or rivers that have worn away the layers of soft rock (such as limestone). Others are formed by the volcanic shifting of the surface rocks, or by eruption of hot lava. The lava develops bubbles of gas inside that, if large enough, can be entered once the lava is thoroughly cooled.
The most common type of cave in the United States is that made by the wearing away of thick layers of limestone. This is done by carbon dioxide (CO2) and water. In Indiana, Kentucky, and Tennessee, such caves are numerous. There are great beds of limestone in these states, with an average thickness of 175 feet.
In many cases, caves reside under the water table or under a source of water. Over time, water seeps through the cracks in the rocks above the cave, and drips down to the bottom of the cave. Each drop of water contains a little bit of limestone, calcium, or other mineral matter. As part of the water evaporates, the mineral matter is left behind. This gradually builds up to form a stalactite on the roof of the cave. Stalactites resemble icicles of rock. However, when the water drips, it does not leave all of the mineral matter behind. What is left pools below the stalactite to create a stalagmite. These look like little mountains of rock, pointing up to where the stalactite is.