The Viking 1 space probe settled into orbit around Mars in 1976, dropping a surface lander in the process. The probe stayed in orbit to monitor the planet, returning thousands of pictures from millions of kilometers away.
Those pictures are sitting in an archive, and sometimes have hidden jewels in them. As Emily Lakdawalla reports, somehow the keen-eyed Daniel Macháček spotted an amazing thing: the shadow of the Martian moon Phobos passing over a dust storm:
Wow! This animation is sped up by a factor of 10, and you can see the tiny moon's shadow slip across the face of the planet. He also has one sped up 40X, and there you can see the slow movement of the dust storm, too... though it's only slow due to distance; I'm sure someone standing on the surface would laugh ruefully at describing the 100 km/hr gusts as "slow". If they could breathe, that is.
Anyway, Emily has more information on the animations. I think this is amazing work, and we'll be seeing more things like this as the planetary (and astronomical) databases get plumbed by the public. A lot of folks out there are very talented at digging out treasures, and equally adept at creating beautiful imagery and animations, too.