Former President of the United States, Richard Nixon, recorded many of his conversations, phone or otherwise, in the White House for his own records. He was far from the first president to do so; Lyndon B. Johnson and John F. Kennedy beat him to that title. What makes Nixon's recordings so unique is the sheer length of material. Unfortunately for Nixon, over 200 hours of his recordings would be used against him for the Watergate Scandal.
Reportedly, over 18 minutes of the final portions of the tape were wiped over. There are various theories as to why this happened. According to Nixon himself, the tapes were wiped by accident by an associate of his named Rose Mary Woods. Nixon was harassed by various members of the media, with many claiming he deliberately erased the audio. It is not entirely known exactly why the tape was wiped.
This, however, has not stopped various conspiracy theory groups from using the wiped audio as "evidence". Theories of what the audio contained ranges from a deal with terrorists to war plans with the Soviet Union. In truth, the audio is more than likely just more incriminating Watergate material, or insignificant phone conversations with his secretaries.
Despite the material being gone, modern day technology is getting closer and closer to being able to reverse the process of an audio-tape being wiped. The reversal process may very well bring this mysterious piece of history to light.