If they're hoaxes or mass hallucinations, then they've affected more than 15 million people around the world.
There were 115 reported sightings in Australia alone last year. But once considered to be flights of fancy, now science is taking the mass reports of UFO sightings more seriously.
It's the greatest mystery in science. Are we truly alone in the universe whether you believe in UFO's and little alien visitors, there's one certainty in the whole debate. There's something weird going on up there.
By definition a UFO is nothing more than an unidentified flying object. Something in the sky you just can't name like an unusual disc photographed in Australia but it's become synonymous with flying saucers, aliens, abductions, weird experiments and the encounters you'll see seem to defy any rational explanation.
It was the 1950's, a largely peaceful and prosperous time despite the growing tensions of the Cold War. In a small town in outback Oregon in America, a farmer and his wife were to make history.
"I looked up and saw this thing in the sky. It was bronze on top and silver on the bottom. I would say it was the size of a big parachute." Evelyn Trent saw it, husband Paul photographed it and it caused a stir in newspapers around the world.
Another photo was taken years before digital technology and doctored photos but skeptics like Robert Shaeffer remain unconvinced.
The Trents are dead now but years on, the debate still rages.
Easy enough to discount simple farmers much harder to discount trained fighter pilots.
Mexican Air Force anti drug pilots were hunting drug runners when they came across something they just couldn't explain.
As the pilots got closer, the objects moved away but their heat signatures were picked up on infra-red, there's still no explanation for this encounter.
Not all UFO sightings happen in isolated farm lands in America. One bizarre apparition was seen in Belgium.
"There is a very large, extremely large triangular craft that could manoeuvre silently in the night sky, very slow very low. The Belgian craft was witnessed by hundreds of people" says film maker James Fox.
"I think it's time for us to realise that we don't know everything and we're not the top of the tree" says Doug Moffett is from UFO Research NSW. He's spent more than a decade trying to unlock the mystery of sightings just like this.
"It's very difficult for people to come up and tell people. It's one thing to say you've seen a craft, it's another thing to say you've interfaced with a being from another planet" adds Doug.
And it's harder to scoff at sightings when they're made by astronauts. Like the two pilots of the Gemini XI space craft who saw something they'd never seen before.
"One of them saw this light that seemed to be flashing... they were travelling at 17,000 miles an hour. This object was obviously travelling also at a very high speed" says a NASA insider.
UFO researchers in Australia are now scouring formerly classified documents, it's called Project Disclosure. Secret files revealing what most of us have suspected for years, that governments knew.
Whatever these UFO's really are, space travellers, natural events, weather balloons, scientists are taking sightings far more seriously. In fact Hong Kong will soon open the world's first school of UF-OLOGY... perhaps the truth is really out there.
Further informationFor more information or to report an experience:
Project Disclosure: www.disclosureaustralia.freewebpages.org
UFO Society: www.ufosociety.com.au