W. Mark Felt (Deep Throat)

For more than 30 years, people speculated about the identity of "Deep Throat", the secret informant of Bob Woodward and Carl Bernstein, The Washington Post reporters who broke the Watergate Scandal in 1972. In 2005, Felt's identity was finally made public. Felt was associate director of the FBI, the second-in-command at the bureau, when he leaked information about then-president Richard Nixon's involvement in Watergate.

While Watergate eventually led to Nixon's resignation and prison terms for two top officials, since the revelation of Felt's identity, historians have debated whether he acted as informant due to ethical concerns, or if it was simply an attempt to get the top job at the bureau. Either way, the decades-long mystery surrounding his identity and the history-making implications of his actions land Felt in the top spot of famous whistleblowers in US history.