A cave is a natural hollow space inside a mountain, cliff, or underground, usually formed over thousands of years as water slowly dissolves rock or as lava flows create empty tubes. Some caves are small enough that you have to crawl through them, while others have chambers as large as cathedrals, with stunning formations like stalactites hanging from the ceiling like stone icicles.

Throughout history, caves have served as shelter, hiding places, and sacred spaces. Early humans lived in caves and painted remarkable art on their walls, some of it over 30,000 years old. During wartime, people have hidden in caves to escape danger. Explorers called spelunkers or cavers study and map cave systems, sometimes discovering underground rivers, rare creatures, or passages that stretch for miles.

The word also works as a verb. When you cave or cave in, you give up under pressure, like finally agreeing to your friend's movie choice after they keep insisting. A roof can also cave in when it collapses inward.