An unidentified submerged object, or USO, is any object or optical or mechanical detection phenomenon of unknown origin observed under water that remains unidentified even after thorough investigation. This particular use of the acronym should not be confused with USO, the popular abbreviation of the United Services Organization. The maritime analog to UFOs, or unidentified flying objects, unidentified submerged objects are often seen by those who study unidentified flying objects as a related phenomenon (ufologists). Likewise, many sightings of USOs constituting "typical" UFOs emerging from the water are seen as a subset of the latter rather than of the former. Although identifying unidentified flying objects and dispelling mistaken ideas about them has typically been important to the military as a national security issue, the comparatively rare USO sightings have not been reported as national security threats nor resulted in the sort of sensation that UFO sightings have often caused .