the.com/underbelly

means The hidden, vulnerable or seedy underside of something — the exposed soft part most likely to get hurt, or the shady part most likely to be hidden.

from Literal first: the soft belly of an animal, unprotected by shell, hide, or armor, versus the tougher back and sides. Hunters and soldiers knew that's where a blade or claw actually does damage — armored beasts and knights alike were vulnerable there. The word crawled from zoology into metaphor by the 20th century, used to describe the vulnerable or disreputable hidden side of a city, an institution, or a plan — the part exposed when you flip the whole thing over and look underneath.