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 dwelling place of people who had no other shelter.
But long after man stopped using caves as homes, ancient people believed many strange things about caves. The Greeks believed caves were the temples of 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.
Today, huge and beautiful caves all over the world are tourist attractions. Caves are deep hollow places in the rocky sides of hills or cliffs. Large caves are called caverns.
Caves are formed in many different ways. Many caves have been hollowed out by the constant beating of the sea waves against the rocks. Some caves appear under the surface of the earth.
These are usually the old courses of underground streams which have worn away layers of soft rock such as limestone. Others are formed by the volcanic shifting of surface rocks, or by the eruption of hot lava.
The most common type of cave in the United States is made by the wearing away of thick layers of limestone. This is done by the action of water containing carbon dioxide. In Indiana, Kentucky, and Tennessee, where there are great beds of limestone with an average thickness of 175 feet, such caves are numerous.
Some caves have openings through their roofs, called sinkholes. These formed where the surface water first gathered and seeped down. Some caves have galleries in tiers or rows, one above another.
Underground streams wind through some caves, though in many cases after a cave has been formed, the streams that once flowed through it may find a lower level and leave the cave dry.
In many cases, each drop of water that drips from a cave roof contains a bit of lime or other mineral matter. As part of the water evaporates, some of this matter is left behind.
t gradually forms a stalactite, shaped like an icicle hanging from the roof. Water dripping from the stalactite to the floor builds up a column called a stalagmite.