Project Blue Book UFO Unknowns
This is the complete list of UFO sightings that the U.S. Air Force was unable to explain:
http://www.projectbluebook.info/exm/701_Unknowns_Part_1/
http://www.projectbluebook.info/exm/701_Unknowns_Part_2/
Project Blue Book was an official U.S. government investigative UFO project. The project that started in March 1952 and was terminated in January 1970 was run by the U.S. Air Force.
Official U.S. Air Force factsheet: “UNIDENTIFIED FLYING OBJECTS AND AIR FORCE PROJECT BLUE BOOK”:
http://www.af.mil/information/factsheets/factsheet.asp?id=188
“Project Blue Book –
An Investigative Project Of The United States Air Force”:
http://www.projectbluebook.us/
Wikipedia article: “Project Blue Book”:
http://en.wikipedia.org/wiki/Project_Blue_Book