A chance meeting in a White House waiting room sparked Bob Woodward's relationship with his Watergate source Deep Throat, the reporter has revealed.
Mark Felt started offering careers advice to a young Bob Woodward
Woodward, writing in the Washington Post, said his "accidental encounter" with Mark Felt happened before he had started work as a journalist.
Mr Felt, a former deputy FBI chief, steered Woodward and his colleague Carl Bernstein towards exposing the scandal.
But his first help came in the form of careers advice in 1970, Woodward says.
The two men met in a West Wing waiting room when Woodward was a young navy lieutenant acting as a courier for an admiral.
Woodward extracted from Mr Felt that he was an assistant director in the FBI - and started quizzing him about this secret world.
"As I think back on this accidental but crucial encounter - one of the most important in my life - I see that my patter probably verged on the adolescent," Woodward writes.
"Since he wasn't saying much about himself, I turned it into a career-counselling session."
Woodward was suffering "considerable anxiety" about his future, he says, and after a long chat with the older man, was given his direct telephone number.
When Woodward later started work as a local newspaper reporter, Mr Felt continued to act as a career counsellor and a mentor.
"We were becoming friends of a sort. He was the mentor... and I kept asking for advice," Woodward says.
As their relationship developed, Woodward says he noticed Mr Felt "was a man under pressure" who did not agree with the way the Nixon administration was operating and "thought the Nixon team were Nazis".
"The threat to the integrity and independence of the bureau was real and seemed uppermost in his mind," Woodward says.
In July 1971, Mr Felt was promoted to number three in the FBI, but in effect became the "day-to-day manager of all FBI matters" as his immediate superior was ill.
A month later Woodward was hired to work for the Washington Post.
At that stage, Mr Felt urged Woodward to never reveal they knew each other, although he was happy to talk to him about FBI and justice department matters, and was his source on a number of subsequent front-page stories.
Then the Watergate break-in happened, which Woodward says became "the moment when a source or friend in the investigative agencies of government is invaluable".
The two men started discussing aspects of the break-in over the phone, but Mr Felt was uncomfortable about those conversations.
Eventually Mr Felt, who had knowledge of espionage and surveillance practices, refused to take phone calls or meet in the open.
The reporters called on Felt 'when they had nowhere else to turn'
The two men agreed to the famed "flower pot signal" which involved moving a flower pot on Woodward's apartment balcony if he urgently needed a meeting.
The signal would mean meeting the same night on the bottom level of an underground garage just over the Key Bridge in Rosslyn, Washington DC - although Woodward says it was never clear to him how Mr Felt could have made the daily observation of his balcony.
"There was no fallback meeting place or time. If we both didn't show, there would be no meeting," he says.
"Felt said if there was something important he could get to my New York Times - how, I never knew.
"Page 20 would be circled, and the hands of a clock in the lower part of the page would be drawn to indicate the time of the meeting that night, probably 0200, in the same Rosslyn parking garage.
"The relationship was a compact of trust; nothing about it was to be discussed or shared with anyone, he said."
Woodward and Bernstein pushed Mr Felt hard as they often had "nowhere else to turn" - but the FBI man showed great patience, he says.
"Felt said I should not worry about pushing him," Woodward writes.
"He had done his time as a street agent, interviewing people. The FBI, like the press, had to rely on voluntary co-operation. Most people wanted to help the FBI, but the FBI knew about rejection.
"Felt perhaps tolerated my aggressiveness and pushy approach because he had been the same way himself when he was younger, once talking his way into an interview with (J Edgar) Hoover (ex-FBI chief) and telling him of his ambition to become a special agent in charge of an FBI field office."
Woodward says Mr Felt's help was invaluable in helping him and Bernstein to understand the "many-headed monster of Watergate".
He writes: "Because of his position virtually atop the chief investigative agency, his words and guidance had immense, at times even staggering, authority."
On Mr Felt's motives, Woodward says he "believed he was protecting the bureau by finding a way, clandestine as it was, to push some of the information from the FBI interviews and files out to the public, to help build public and political pressure to make Nixon and his people answerable".
Woodward adds: "He had nothing but contempt for the Nixon White House and their efforts to manipulate the bureau for political reasons."