Caving is the sport of going into caves deep under the ground. People walk, climb, crawl, and sometimes swim through dark rock passages. It is the activity; the person who does it is a caver. In some places, especially the United States, people also say spelunking for the same sport, though many cavers themselves prefer caving. It is not a visit to a tourist cave with a path and electric lights. The word can also be the -ing form of the verb cave, including the very different phrasal verb cave in, which means to fall in or to give way. That is a different meaning that happens to share the letters.