photo of a purported UFO over Passaic, New Jersey, from an FBI document.]]An unidentified flying object, or UFO, is any real or apparent flying object which cannot be identified by the observer and which remains unidentified after investigation. The term flying saucer is also sometimes used, although the two terms are not genuinely interchangeable. Reports of unusual aerial phenomena date back to ancient times (see Ancient astronaut theories), but modern reports and first official investigations began during World War II with sightings of so-called foo fighters by Allied airplane crews and in 1946 with widespread sightings of European ghost rockets. UFO reports became even more common after the first widely publicized United States sighting in 1947. Many tens of thousands of UFO reports have since been made worldwide. Perhaps the largest database of sightings is UFOCAT with 109,000