For decades people around the world have claimed they've seen UFOs hovering in the skies. It's a mystery that science has been unable to solve. Much of the reporting on this subject holds those who claim to have seen UFOs up to ridicule. "UFOs… Seeing Is Believing" examines the phenomenon in today’s world. The 90-minute special includes interviews with scientists searching for proof of life beyond earth and UFO witnesses who claim aliens are already here. Building on the original Peter Jennings special in 2005, David Muir reports on new sightings, and examines NASA’s current search for life on Mars. "UFOs… Seeing Is Believing," to be broadcast in HD, airs on a special edition of "Primetime," TUESDAY, SEPTEMBER 16 (9:31-11:00 p.m., ET) on ABC.

The program follows the entire scope of the UFO experience, from the first famous sighting by Kenneth Arnold in 1947 to the present. Muir reports on a recent sighting in Stephenville, TX, where multiple witnesses reported seeing enormous lights moving in strange configurations on the evening of January 8, 2008. He interviews some of the most credible witnesses of the sighting and a radar expert who evaluated their claims and found something surprising in the data. The special draws on interviews with police, pilots, military personnel, scientists and ordinary citizens who give extraordinary accounts of encounters with the unexplained. And professional skeptics speak out about UFOs.

The special also reports on the most recent advances in the search for life on other planets. This summer, NASA's Phoenix Mars Lander, which tests soil on the planet, found conclusive evidence of water ice on Mars. Viewers will hear from the scientists on the project about the possibility of finding life on Earth’s next-door neighbor, and how that could change our understanding of the universe.