An unidentified submerged object (USO) is an object which can be detected underwater, but not identified and often not confirmed. In addition to being found entirely underwater, unidentified submerged objects have also been described in transit, hopping into or out of the water, sometimes multiple times. Like unidentified flying objects, their airborne cousins, USOs are a topic of great debate and discussion in some communities, with some people firmly maintaining that they are evidence of alien life or other supernatural phenomena, while others suggest that there are perfectly mundane explanations for such objects.
There are a number of ways in which an unidentified submerged object can be observed. In some cases, people actually make visual contact, as in the case of an object which appears to burst through floating sea ice, or skim along the surface of the water. In other instances, mechanical detection methods like radar may reveal anomalies in the water which do not match with any known objects in the ocean. Unidentified submerged objects can also be identified audibly, as in the case of mysterious noise which filter through microphones placed on the ocean's floor.
The ocean is a very big place, and it's also a very mysterious place. Many parts of the ocean are largely inaccessible to people because they are so hostile, with total darkness, extreme cold, and deadly pressure being serious obstacles to human interaction. When humans do venture to the floor of the ocean in specially designed submarines or through the assistance of remotely operated vehicles equipped with cameras, they often see a number of fantastic things, like the huge tube worms which congregate around hydrothermal vents in the ocean floor. One could be forgiven for mistaking such curious creatures for an unidentified submerged object, especially since researchers are constantly finding new species in the ocean.
One could argue that most documented cases of unidentified submerged objects are probably just instances in which people saw plant or animal life from the depths of the ocean floor and didn't recognize it because it was totally unfamiliar. Some deep-sea fish, for example, really do look like aliens, with some species even generating light to lure prey. In the case of objects which appear to move in and out of the water, tricks of the light, water, and environment could be major factors in these sightings, especially since such sightings have not been verified, and crew members have often later admitted that they were exhausted or stressed.
Some oceanographers are fond of saying that we know more about the surface of the moon that we do about the depths of the ocean, and this could explain the truth behind the unidentified submerged object. The mysteries of the ocean do not appear to have captured the attention of the paranormal community in the way that deep space has, which might also explain the comparative rarity of unidentified submerged object sightings. Many nations have also invested a lot of money in oceanographic research, which could also contribute to such sightings; the mysterious objects in and around the water may, for example, be experimental watercraft.