The term "cavern" refers to a large underground chamber which is often formed by natural processes like weathering and erosion. Some synonyms for cavern include cave, grotto, and underground cavern. Another alternative is to use the word cavity, which is often used to describe hollow spaces in a rock formation. Additionally, some other synonyms for cavern could include catacomb, cellar, crypt, or subterranean vault. These words all imply spaces underground or within a rock structure. Overall, the use of synonyms helps to add variety and interest to written descriptions, while still conveying the same basic meaning.