On July 2nd 1947, a UFO propelled at supersonic speed by electrogravity
was struck down by lightning over a ranch near Roswell, New Mexico. The wreckage was scattered for 2 miles or so, hence why a lot of witnesses did not see the aliens.
The military employee who told the media was forced to take it back the next day and say it was a weather balloon. However the wreckage was not consistent with a weather balloon. In recent times, many of the witnesses have testified that what crashed at Roswell was not of this Earth.
There is also claims that technology discovered on the crashed UFO has been integrated into human society. Transistors, integrated circuit chips, lasers, accelerated particle beams, and kevlar were supposedly all present on the UFO, before humans took them to the lab and 're-invented' them. A lot of even more advanced technology has yet to see the light of day.
The Roswell incident also helped to fuel the conspiracy known as the Cold War