underbelly

1 a social sphere that exists outside of the mainstream unable to get work in mainstream films, the actor ended up in Hollywood's underbelly—the porn industry Synonyms demimonde, demiworld, half-world, netherworld, underworldRelated Words abyss, depths

2 the side or part facing downward from something the underbelly of the old submarine is in bad shape Synonyms bottom, underbody, underpart, underside, undersurfaceRelated Words belly, sole, toe; base, floor, foot, ground, seat, underpinning; undercarriageNear Antonyms acme, apex, climax, crest, crown, culmination, height, high-water mark, meridian, peak, pinnacle, roof, summit; cusp, head, point, tip, tip-top, zenithAntonyms face, top

3 a vulnerable point the vast, thinly guarded southern border was obviously the underbelly of the nation's line of defense Synonyms back, chink, jugular, soft spot, Achilles' heelRelated Words downfall, ruin, undoing; hamartia, tragic flaw