un‧der‧bel‧ly [singular] literary
the unpleasant parts of a place or society that are normally hidden
photographs that capture the underbelly of the United States - its poverty, its injustice, and its alienated underclass
the weakest part of an organization or a person's character, that is most easily attacked or criticized
They needed to find the soft underbelly of their opponents.
the lower part of an animal's body, including the stomach