5 Edward Snowden
As with Manning, opinions greatly differ on Snowden's character. However, his damage to U.S. intelligence agencies remains indisputable. As a private contractor for the National Security Agency, Snowden acquired a damning collection of documents on how the NSA indiscriminately conducts online surveillance on millions of Americans, raising concerns under the Fourth Amendment. After fleeing the U.S. for Hong Kong, Snowden went public with his allegations.
4 Bradley Manning
Some consider him a traitor. Others call him a hero. Either way, Pfc. Bradley Manning roiled U.S. foreign policy by supplying more than a quarter-million diplomatic cables to the Wikileaks website. The breadth of the disclosure was stunning. The cables spanned decades and originated from dozens of U.S. embassies all over the world. As of July 2013, Manning faced a trial by a military court and potentially a long prison term along with additional federal charges for espionage, theft and computer fraud.
3 Karen Silkwood
Union activist Karen Silkwood worked on a minimum-wage assembly line at the Cimarron Plutonium plant in Oklahoma. Though hardly a nuclear physicist, she quickly identified unsafe conditions at the factory, including radioactive leaks and falsified inspection reports. After she reported her findings to the Atomic Energy Commission, someone may have retaliated. Silkwood alleges that the plant intentionally exposed her to lethal levels of plutonium, although the company has issued the unlikely counterclaim that she exposed herself on purpose to make them look bad. In 1974, she died in a mysterious car accident on her way to an interview with a journalist from The New York Times.
2 Daniel Ellsberg
In the late 1960s, military consultant Daniel Ellsberg became increasingly disillusioned with the Vietnam War. After leaving the Department of Defense in 1967, Rand Corp. hired him to help draft a top-secret report on U.S. policy in Indochina that eventually totaled 7,000 pages in 47 volumes. With the assistance of colleague Anthony Russo in 1971, Ellsberg photocopied the entire collection of documents and sent it to The New York Times. Known as the Pentagon Papers, these documents strengthened opposition to the war and heralded increased scrutiny of U.S. intervention abroad.
1 Mark Felt aka Deep Throat
Whistle-blowers come and go, but the legend of Deep Throat endures. The anonymous source supplied crucial information to the Washington Post that resulted in a series of sensational articles about the break-in at the Democratic National Committee headquarters in the Watergate complex. Though the attempted burglary seemed insignificant at the time, Deep Throat helped journalists Bob Woodward and Carl Bernstein connect the dots of a government-wide conspiracy that led all the way to the White House. Their revelations forced President Nixon to resign. Decades later, Vanity Fair magazine disclosed the whistle-blower as former FBI Associate Director W. Mark Felt.