Unidentified Submarine Object (USO) is a term used to describe an unidentified object seen below the surface or an amphibious UFO. Ship logs contain numerous reports of unusual lights seen on or beneath the ocean surface, particularly during the latter half of the 19th century and the first half of the 20th century. In some instances, the objects were seen entering or emerging from the water. One of the most common descriptions is that of the revolving luminous wheel with spokes of light radiating from its centre. The wheels sometimes measured hundreds of yards in diameter. Supporters of the Submarine hypothesis conjecture that the lights are emitted by vehicles of an advanced submarine civilisation or by UFOs which utilize underwater bases. It has also been suggested that UFOs, even if of extraterrestrial origin, might contain water, rather than air or some other gas.
Proponents of the Ancient Astronaut hypothesis believe that the earliest reference to a USO is found in the Biblical story of Jonah. The whale which allegedly swallowed him might have been a cigar-shaped USO, resembling a modern-day submarine in appearance.
Marine biologists point out that many glowing lights in the ocean can be attributed to phosphorescent plant and animal life. The tropical seas carry dense blankets of single-celled luminous planktonic organisms which glow when stimulated mechanically, as by the movements of the waves. Some flash brightly. The single-celled Cypridina Noctiluca, when distributed by a beam of light, responds by ejecting a luminous cloud in the water. Luminous crustaceans, especially copepods, are widely distributed throughout the world. Some live on the surface, while others live in the ocean depths. Other organisms which create patches of light in the sea are jellyfish and other coelenterates and ctenophores.
The study of marine phosphorescence has not provided the answer to all reports of USOs, particularly those where objects have been seen entering and emerging from water. The Question of there identify remains an open one.