underbelly

noun3

1. •
   The underside of an animal.

   • We identified the species of the snake by its blue-grey underbelly.
2. •
   (figurative) The side which is not normally seen, normally a dark, immoral place.

   • The area by the canal is the underbelly of town – not something you’d see in the guidebooks.
3. •
   The underside of any thing.

   • A jolt told me that the Nautilus had bumped the underbelly of the Ice Bank, still quite thick to judge from the hollowness of the accompanying noise. — 1993