... doesn't mean they're not out to get you: Chemtrails

From Wikipedia:

The accepted theme is the belief that a chemical and/or biological agent of some sort is being released. The term "chemtrail" should not be confused with other forms of aerial dumping (e.g. crop dusting, cloud seeding or aerial firefighting). It specifically refers to systematic, high-altitude dumping of substances such as barium oxide, trimethylaluminium, liquid propane, and mycoplasmas for disclosed purposes of geoengineering resulting in the appearance of these observed chemtrails.

Among the suggested rationales of the chemtrails are: atmospheric and weather modification, biological warfare, mind control or occult purposes. They are also theorized to be part of a system to counter the effects of global warming, to create a cheap wireless communications network for the military (by making the air more electrically conductive to microwave and radar signals), or to create a more sophisticated radar system with applications to both national defense and scientific study, such as those described within the HAARP project and the VTRPE project.

Naturally, NASA denies it ... or is that just what they want us to think?