Here's one that took the U.S. public by surprise. Robert Hanssen, a 25-year FBI agent and church-going family man, was also a long-time double agent for the Soviet Union.
Hanssen worked as a liaison between the FBI and the office responsible for tracking the identities of spies working in the U.S. By the early 2000s, investigators believed he had spent 20 years sharing state secrets. They suspected he'd revealed the identities of dozens of Soviet agents working for the U.S. and led to the killing of several.
And Hanssen didn't stop there. He fed the Soviet government U.S. plans, including the procedures for dealing with and retaliating against a Soviet nuclear attack. Although he was paid $1.4 million and some diamonds for his efforts, colleagues said he seemed to have been more motivated by playing the spy game than by greed.
Hanssen was caught when the FBI paid a former KGB agent to disclose the identity of the mole operating in the FBI. In 2002, Hanssen pled guilty to 15 counts of espionage and conspiracy. He was sentenced to life in prison [sources: CNN, New York Times].