Crop-dusting use small airplanes to deliver pesticides to protect valuable crops. In recent decades, chemtrails have caused quite a stink among conspiracy theorists who believe that certain airplanes are leaving behind more than we bargained for. Specifically, we're talking about trails of biological and/or chemical agents designed to harm people who are unlucky enough to be in the way. Scientists have repeatedly explained that these trails (which in their legitimate form are actually known as contrails) are nothing more than vapor trails caused by normal reactions in aircraft engine exhausts. The size and appearance of these trails can vary based on atmospheric conditions, like humidity and temperature. Chemtrail opponents repeatedly deny the validity of these claims, insisting that humans are being unwittingly poisoned.
So what's the point of poisoning your nation's own people? Advocates of this theory claim that chemtrails are an effort by the U.S. government to keep the population under control, and that these toxic agents are also a result of military weapons testing [source: Stone]. If we all start wandering around like mindless zombies we'll have an idea of why and how it's happening, at least.