Moaning Cavern is located on the western slope of the California Sierra Nevada Mountains in Calaveras County. It is a marble cavern estimated to be about one million years old.
There are two natural entrances to the cavern. Both are narrow passages that drop into the main chamber where the cavern opens up as a large room. One entrance is a vertical drop that was the only entryway into the cavern until its development. The other entrance was a narrow fissure that was found in 1920 and enlarged into a diagonal passage that makes access easier to the cavern.
The main room is over 165 feet (50.3 meters) high, roughly 132 feet (40.2 meters) long and has varying widths up to 102 feet (31.1 meters). The floor of this room is covered with massive blocks of breakdown, which are mixed with the reddish clay soil known as terra rosa. Under this floor are several small passages. Some of these lead to deeper parts of the cavern where one of the rooms had a deep pool of water. The farthest radiation of passage from the entrance is approximately 460 feet, which is the total depth of the cavern system.
Because a cavern is not subjected to the outside elements, the temperature remains constant throughout the year: between +59-62º F (15-17º C). The humidity averages 85%. The cavern is still considered active or “wet,” as water is found dripping throughout the cavern during the winter.
Decorative formations found in caves are called speleothems. There are several minerals that formations can be made of depending on the surrounding area of a cave. The most common solution caverns are limestone, which have calcite formations. Since Moaning Caverns is made of limestone, its formations are calcite.