Unexplained Mystery #3: UFOs and Area 51
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.