I saw some Mammatus Clouds a few years ago. It was at twilight and just before the heavens opened. The only camera available was a video camera and this is a frame taken from that. Could be better quality but the clouds still show OK..
I saw these clouds moving across the Moon and thought they looked a bit spooky..
Edit: I guess it looks like the the clouds are behind the Moon but the photo is unedited. It must be because the Moon was so bright that it shone through the underside of the cloud.