Roswell

In July 1947, a rancher discovered weird debris in his sheep pasture outside of Roswell, New Mexico and this sparked an ongoing UFO controversy. Locals believed that this debris was remnants of a flying saucer, and the government was just trying to cover it up by saying the pieces were from a weather balloon. The debris was described as looking like pieces of tin foil, but if someone were to crumple it up, it would unfold itself and show no signs of creases. It was obviously unlike anything anyone has seen before, so when the government denied the possibility of extraterrestrial products by saying it was from a simple weather balloon, everyone automatically assumed they were trying to cover it up. Fifty years later, it was released that this debris was from a highly classified project called "Project Mogul." The government was testing new balloons that were developed after World War II to possibly pick up sound waves high in the Earth's atmosphere to eavesdrop on possible nuclear plans by the Soviet Union. Even though this was released, people still hold onto the UFO story today and frequently visit the crash site and Roswell's museums.

An element of thought here would be that everyone, even local officials, assumed that the debris was related to extraterrestrial activity just because it was something no one had ever seen before. Since the project was so classified, the local air force base did not even know it was going on, which led to more UFO assumptions. Apparently, the government even threatened locals who witnessed the debris to never speak of this, and so people just thought it was because the government did not want a nationwide frenzy after finding out the aliens really do exist. Years later, we now know that it was simply just because they did not want word getting back to the Soviet Union that they were developing eavesdropping equipment.

http://www.history.com/topics/roswell