Red rain could prove that aliens have already landed.
http://en.wikipedia.org/wiki/Image:Red_rain_Kerala_TEM.png
http://en.wikipedia.org/wiki/Red_rain_in_Kerala
http://observer.guardian.co.uk/world/story/0,,1723913,00.html
http://education.vsnl.com/godfrey/
http://www.earthfiles.com/news/news.cfm?ID=1129&category=Science
