Souterrain is a French word that refers to an underground room or space. This word has several synonyms depending on its usage. If you're referring to a secret underground room used for hiding or storing things, then some synonyms include basement, cellar, dungeon, crypt, and catacomb. On the other hand, if you're referring to a living space or apartment that is partially or fully underground, then some synonyms are subterranean or underground dwelling, terrace level, or garden level. In the context of military fortifications, some synonyms include underground bunker, shelter, or hideout. Whatever context you're using it in, there are several words you can use instead of souterrain to add variety and interest to your writing.