Project Blue Book — Final Case Disposition Summary
U.S. Air Force
1976 (National Archives)
1969
The closing ledger of the Air Force's 17-year UFO investigation: 12,618 sightings logged, 701 left unidentified, and a recommendation that the program be shut down.
Why it was classified
Individual case files contained radar performance data and interceptor scramble procedures that revealed continental air defense readiness.
Why it matters
Those 701 unexplained cases are still the baseline every modern UAP program is measured against.