SIGHTINGS OF UNIDENTIFIED OBJECTS INCREASED, AND IN 1948 THE U.S. AIR FORCE BEGAN AN INVESTIGATION OF THESE REPORTS CALLED PROJECT SIGN. THE INITIAL OPINION OF THOSE INVOLVED WITH THE PROJECT WAS THAT UFO'S WERE MOST LIKELY SOPHISTICATED SOVIET AIRCRAFT, ALTHOUGH SOME RESEARCHERS SUGGESTED THAT THEY MIGHT BE SPACECRAFT FROM OTHER WORLDS, THE SO CALLED EXTRATERRESTRIAL HYPOTHESIS. WITHIN A YEAR, PROJECT SIGN WAS SUCCEDED BY PROJECT GRUDGE, WHICH IN 1952 WAS ITSELF REPLACED BY PROJECT BLUEBOOK, HEADQUARTERED AT WRIGHT PATERSON AFB IN DAYTON, OHIO. FROM 1952-69, PROJECT BLUEBOOK COMPILED REPORTS OF MORE THAN 12,000 SIGHTINGS OR EVENTS. EACH OF WHICH WERE ULTIMATELY CLASSIFIED WITH AN IDENTIFIED CAUSE OR UNIDENTIFIED. THE CATEGORY - UNIDENTIFIED - COMPRISED 6-10% OF THE TOTAL, INCLUDING CASES FOR WHICH THERE WAS INSUFFICIENT INFORMATION TO MAKE AN IDENTIFICATION WITH A KNOWN PHENOMENON.