The Chunnel comes to mind. How do we really know that there’s a tunnel under the English Channel? Maybe the pictures are faked. Maybe all the people who worked on the project lied about it.
I did some research on the subject. You can find the results of my research here at my Great Chunnel Hoax web page.
How do we know that there is really a Hoover Dam? Any other hoaxes that can be revealed right here on the SDMB? We are fighting ignorance, after all.