According to this conspiracy theory, a manned mission to Mars took place in 1973 and no-one was told about it.
This is impossible, and yet, there it is...captured in a photo on a NASA website.
What would an ancient alien base look like on Mars? Well this photo from the NASA website clears up that question. I found an alien base at the bottom of the photo. The thing that really stands out is the square structure with a tunnel connecting a pyramid area. What this tells me is that the aliens that created it did not just roam around the planet freely but instead used the tunnels to stay safe.