Maybe the tavern's cellar is really spacious? And has stalactites for some reason?
Being a bit serious, it could be a cave being used as a wine cellar or cheese storage (explains the rats) or something similar for a tavern. Although likely not lol.
(Which gives me an amusing idea for a campaign. The tavern keeper wants you to clear his cellar of rats or anything else down there. Except his cellar is a cave that he uses for storage. Which leads to further caves, with more and more deadly monsters the further you go in. There are dragons down there! Whole drow cities! And the tavern keeper won't pay you until you've cleared "everything down there" out.)