For me it all started with a TV special based on Erich Von Daniken's theories. It was entitled IN SEARCH OF ANCIENT ASTRONAUTS. I devoured it and then sat in front of the TV with my reel to reel tape recorder and recorded the rerun. Then they came out with a feature length documentary...or two...or three. Almost overnight, these documentaries of questionable theories, almost always narrated by Rod Serling or Orson Welles or someone with a similarly influential voice, became an entire sub-genre.
These two were my favorites. After that, the novelty began to wear off.
Eventually, the trend led to TV's long-running IN SEARCH OF..., narrated by Leonard Nimoy in what became his major post-STAR TREK gig.