i know there is a large network of people who are all into the chemtrails thing, but i'm also pretty sure it is yet another hoax or conspiracy theory. not that planes are pure, they do pollute, but they don't give off anything other than byproducts of combustion of jet fuel. oh, and of course there are people who are doing cloud seeding for precipitation but that's about it.
White lines in the sky that look like contrails, but theorists claim are some kind of illicit spraying. My favorite pseudoscientific conspiracy theory gets a forum of its own.
www.metabunk.org