British Telly

Broadcast as an episode of Anglia TV's Science Report, it was supposed to have been broadcast in April 1st 1977 but got delayed until June.

It describes, with a straight face, a secret space program to colonise the Moon and Mars. Unfortunately, like the X-Files a decade and a half later, some would take it as fact (or fiction trying to leak secret information). In particular Milton William Cooper picked up the ball and ran with it, his work ultimately becoming one of the sources of inspiration for the X-Files.

There was an official tie-in book (that I own) as well as a handful of conspiracy theory books looking at it in far too much detail. The idea of a Secret Space Program (and Breakaway Civilisations) have continued to metastasise into increasingly outlandish claims.