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. Reports of unusual aerial phenomena date back to ancient times, but reports of UFO sightings started becoming more common after the first widely publicized United States sighting in 1947. Many tens of thousands of UFO reports have since been made worldwide. Many more sightings, however, may remain unreported due to fear of public ridicule because of the social stigma surrounding the subject of UFOs and because most nations lack any officially sanctioned authority to receive and evaluate UFO reports.