Roswell
The Battle of Los Angeles
1952 Washington DC UFO sighting
These 3 events got the Government actually involved in researching UFO's. But the official statements about the events have weak explanations. If you research enough, there is an old government document that actually says to deny the existence of UFOs. I read it a few months ago and was pretty stunned.
I don't know if this is considered a conspiracy, but I've been thinking about what if there was an advanced race of humans that pre-dates us.
This other thing I've been thinking about, is a long long time ago, there was a pole shift, and that is how Antartica got into it's present place. But before the pole shift, maybe there was an advanced civilization on Antartica and there was an ancient city.
Now, in present time, that ancient city or pyramids what have you, are hidden under ice. And the Government covers it up.
http://dsc.discovery.com/news/briefs/20041101/leaves.htmlHere is a link that shows a petrified leaf found in Antarctica because millions of years ago, there were forests on Antarctica. There was also all kinds of shit:
http://en.wikipedia.org/wiki/Antarctica