A great answer to a question about flouridation-focused conspiracy theories noted that this is an example of a conspiracy theory that seems fairly impervious to reason, and just doesn't die out. This got me to thinking: aren't all conspiracy theories modern Americans would be aware of like that? People confront Buzz Aldrin claiming the moon landing was faked. JFK assassination conspiracies haven't died out. Heck... people still claim that the Earth is flat, and, well, what conspiracy could be older or dumber than that? And yet there it is...

So the question: are there examples of once popular conspiracy theories that have actually "died out" and been lost to the dustbin of history? (At least until one of you brilliant folk pull it back out again, if only briefly--don't forget to put it back, please!)