un·der·bel·ly 
(ŭndər-bĕl′ē)
(ŭndər-bĕl′ē)n. pl. un·der·bel·lies
1. The soft belly or underside of an animal's body. Also called underbody.
2. The bottom side or part: scraped the car's underbelly going over the curb.
3. The vulnerable or weak part or aspect: "What if quantum theory is wrong? This is the soft underbelly of the whole project of quantum gravity" (Lee Smolin).
4. The corrupt or immoral part or aspect: "[a novel] that most effectively exposed the decadent underbelly of corporate high tech" (Theodore Roszack).