The word 洞穴 is formed by combining two characters, 洞 and 穴, which both independently carry the core meaning of a hollow or cavity in the earth. This construction visually and conceptually doubles down on the same essential idea, merging the image of a deeper, often water-related hollow (洞) with that of a hole or den (穴) to create a single, reinforced concept of a substantial, enclosed underground space, a cave or cavern. It is a classic example of semantic compounding where two synonyms are paired to form a more vivid and specific term.