As a lifetime Federal Bureau of Investigation agent and No. 2 to J. Edgar Hoover, Mark Felt was not exactly an ordinary man, but he was, it seems, a highly unlikely candidate to topple a presidency. Felt was the man behind Deep Throat, the Watergate whistleblower who led Bob Woodward and Carl Bernstein to the explosive truth behind that break-in. He lived only as a shadowy mystery in the popular imagination until he gave up his long-held secret in 2005, a few years before he died. By then what he represented had already transcended anything an actual human could live up to.