UFO sightings have been associated with so-called alien abductions, in which people say that they have been transported to an alien spacecraft and subjected to a variety of physical examinations -- even alien cross-breeding experiments.
UFOs have also been linked to crop circles, strange and sometimes unexplained patterns that form overnight in fields.
UFOs have been blamed for animal mutilations -- horses, goats, cows, deer and other creatures found drained of blood and with their organs removed. The alien theory arises because typically no tracks are found around the animal to indicate that a human was responsible.
During a UFO sighting, strange phenomena are often reported, such as radio and TV interference or car-ignition failure. Many UFOs leave strange calling cards, such as indentations in the ground; burned or flattened vegetation; spider-web-like strings that hang from telephone poles and trees and disintegrate at the touch; and chunks of unidentifiable debris.
So, are they really alien spacecraft piloted by extraterrestrial beings, or are they terrestrial objects that just haven't been properly identified? The question has raised a good bit of controversy, pitting those who believe in UFOs against those who say they need to see more scientific UFO evidence.
Surveys show that the overwhelming majority of Americans believe that intelligent life exists elsewhere in the universe. More than 60 percent of respondents to a 1997 CNN poll said they believe aliens have contacted humans; approximately 50 percent said they think aliens have abducted humans; and 80 percent believe the government is hiding something about alien contact.
People who claim to have seen UFOs are confident that what they've seen is real. They say these alien beings have come to Earth to study the human race, create a new hybrid species or simply to communicate with humans.
But skeptics say there is a startling lack of real scientific evidence to prove -- or disprove -- the UFO phenomenon. They argue that the majority of UFOs turn out to be identifiable phenomena -- everything from weather balloons to meteor showers to hoaxes.