The first reported UFO sighting happened in Texas in 1878, when a local farmer reported seeing a large, dark, circular flying object flying "at wonderful speed".
Another early sighting occurred in the UK in 1916, when a pilot reported seeing a row of lights that rose and disappeared into the sky.
As with crop circles, after UFOs were popularized by science fiction in the 1950s, the number of sightings went through the roof. Theories to explain the paranormal phenomena range from the good old Extraterrestrial Hypothesis (aliens from another planet) to the Interdimensional Hypothesis (aliens popping over from a parallel universe).
Although I do heartily believe that there is other intelligent life out there (and tons of it - the universe is so mind-bogglingly huge, perhaps even infinite) I don't believe aliens are visiting us, let alone in 1950s-style flying saucers.
In a vast number of sightings, the so-called UFOs can be explained as airplanes, helicopters, weather balloons, comets, meteors and even the five planets which can be seen with the naked eye.
This is supported by the fact that in photos and videos, many UFOs are debunked as being dust on the camera lens or simply all-out hoaxes (which is easy to do now with PhotoShop). Meanwhile, night-time alien abductions are attributed to the hallucinogenic effect of sleep paralysis.
Meanwhile, UFO conspiracy theories center around Area 51 in Nevada, about 90 miles north of Las Vegas. The site houses a large airbase that was selected in the 1950s for testing of a U-2 spy plane. It has since become America's testing ground for secret "black budget" aircraft before they go public.
