Secrets of UFOs

Crop circles have been reported globally for over a century. Some are clearly hoaxes. Others are inexplicable with no evidence of human activity. Why do crops undergo changes at the cellular level? What are the strange lights seen over crop circle fields?

In July, 1947 the Roswell Army Air Field in New Mexico issued a press release stating that base personnel recovered a crashed "flying disc". The Air Force later recanted the announcement.

December 9, 1965: Hundreds witness an object crash near Kecksburg, Pennsylvania. The military recovered the object saying it was a meteor.

It's a mysterious complex of buildings and runways. Photographed by satellites. Documented on magazine covers. Signs authorizing the use of deadly force surround the Area 51 base. Unmarked helicopters are on constant patrol. What are they guarding?