A cave is hollow space underground that's big enough for a person to walk or crawl into. People who explore caves often wear battery-powered headlamps.
You might see a small cave in the side of a cliff as you hike up a mountain, or swim into an underwater cave when you go snorkeling. In either case, a cave is a deep, carved-out chamber that opens above ground — or in the water. People who regularly explore caves are called spelunkers, or cavers. The Latin root word is cavea, "hollow."