Criminals look to completely untraceable guns to get away with crimes; uncovering the "ghost gun" that is potentially invisible to law enforcement.

The illegal, multibillion-dollar global sex trade includes brothels, massage parlors and online escort agencies and is one of the oldest professions. Take a look inside this illegal trade where pimps treat women as commodities.

For migrants crossing illegally from Mexico to the United States, surviving in the desert alone can be a brutal experience.

Millions of Americans love to gamble, but some forms of gambling are restricted or illegal, pushing them underground where the rules are very different.

Anything and everything can be faked: watches, purses, shoes, electronics, smartphones. Go underground to see the breathtaking enterprise and expertise of counterfeiting — an economy that turns over an estimated $500 billion a year.

Go inside the underground economy of knockoff pharma, where the demand for fake drugs continues to grow and buyers can find an abundance on the Internet, in the pharmacies and on the streets.