What exactly is spelunking, and where is it practiced?
Spelunking is the practice of exploring and climbing in underground caves; Caving is the more modern and accepted term today. Many spelunkers are amateurs exploring for fun, while professional organizations exist to map new and unexplored systems. Spelunking is similar to, and shares equipment and terminology with, mountain and rock climbing, but occurs underground instead of above it.
Spelunking takes place in single caves and cavern systems which used to contain water, oil, or magma. Most professionals recommend spelunking in groups, at least at first. Caverns that contain water can be extremely dangerous; spelunking in these caves is called "cave-diving" (the term is often used incorrectly to refer to spelunking in dry caves) and is done with SCUBA and other specialized equipment for safety. Many large cave systems have been extensively mapped and some are equipped with walkways and platforms for tourists.