Beginning in the 1960s, thousands of UFO landings, flyovers, and collisions have left scientific evidence behind for study. These "trace cases" offer up some of the most convincing evidence of the UFO reality.Beginning in the 1960s, thousands of UFO landings, flyovers, and collisions have left scientific evidence behind for study. These "trace cases" offer up some of the most convincing evidence of the UFO reality.