Conspiracy theories in history (LC)

[kimchi]

Mods - This was the only forum I thought I might get reasonable informed replies.

When I google to find info on politics or finance, I'm often directed to conspiracy theorists' websites. While some are interesting, others frightening, and a few hilarious, there seems to be more "conspiracy theories" now than ever before.

I assume the internet and the availablity of information sharing coupled with lack of censorship is largely responsible for their growth and popularity.

I was wondering what large-scale conspiracy theories were prevalent in the past (ie-before the internet).

A few I can think of off the top of head:
-The deaths of JFK, Marilyn Monroe, Princess Diana, Bruce Lee
-Crash of '29
-The purchase of world-changing patents and subsequent supression of development. (eg-oil companies buying and shelving patents on cheap renewable energy souces, or drug companies doing the same with cures etc.)
-Roswell

I'm more interested in theories that were prevalent in historical context rather than contemporary historians and researchers digging up new information on historical events (eg- The DaVinci Code, Battle of Hastings etc.)

Could you please list any famous conspiracies from history?