Though the top of Jonah Town is a relatively law-abiding place, the inside of Jonah Town is a hive of scum, villainy and filth- with extra emphasis on the last part, as the underground of Jonah Town (aptly named "Gullet") is right inside the giant whale that carries the whole town. Down below lies a massive syndicate of criminals, all led by the powerful Frogfather, who commands his various soldiers with an iron fist. This is the perfect place for pirates to hide out- after all, the Armada wouldn't think anyone would be crazy enough to have a hideout inside of a giant whale!
Among the residents of Gullet is a brilliant inventor by the name of Lucius Fox- he's more than capable of providing technological aid to any pirate that needs it. Unfortunately, this has put him on the most wanted list of the Armada, so Lucius never sets foot outside of Gullet; he doesn't even go outside!