There is an average 1100 UFO sightings in the USA (2012). It may seem through the roof; i was shocked when i found this out. There are alot more than you think. They are keeping a close eye on us; of couse, no one has any idea as to why, but there are predictions. This year alone, the USA had more reported UFO than any other year.
Theres been a huge sighting in phoenix arizona a few years ago(google phoenix lights) and of course the roswell incident and the shooting of an aircraft in LA in the 40's. i think those are the most notorious