Origin

The character 洞 combines water (氵) on the left with a sound hint (同) on the right. The water radical suggests this character relates to water or wet places, which fits the meaning of a cave or den—often damp spaces where water flows or collects. The right component 同 doesn't add meaning here; it's purely a sound hint telling you roughly how to pronounce the character. Over time, the shape came to represent the idea of a hollow space or excavation, especially one associated with water.