Underbelly (noun)

1. The weakest or most vulnerable part of something.
2. The hidden or less visible part of something, especially something unpleasant or illegal.

Origin:

Early 20th century: from under- + belly.

Examples:

1. The city's underbelly was filled with crime and poverty.
2. The underbelly of the internet is a dangerous place.
3. The company's underbelly was exposed during the scandal.
4. The underbelly of the beast was vulnerable to attack.
5. The underbelly of the economy was in a recession.