Humans have always observed their environment — from their immediate surroundings, to the skies, and beyond. While technological advancements have increased our powers of perception, the naked senses remain extremely limited. Commonplace observations by the unaided senses, notably sightings of Unidentified Flying Objects, or UFOs, can be very difficult to explain. Our understanding of these phenomena is limited to speculation, anecdote, and the imperfect powers of human perception and recollection.There have been documented UFO sightings in the United States since the late 19th century, before manned flight. Today, tens of thousands of UFO sightings are reported nationwide every year. Depending on weather conditions and the amount of time spent outdoors, UFO sighting levels vary by state.