Each year the U.S. economy is flush with more than $500 billion generated in illegal transactions. From sex, weapons and drugs to gambling and counterfeit merchandise, nothing is off limits and seemingly everything is for sale. In "Underworld, Inc.," National Geographic Channel is granted unprecedented access to this "free" enterprise. Workers, suppliers, sellers, customers -- and law enforcement personnel trying to keep it in check -- share their experiences of an underground industry that is brutal, exploitive and corrupt.