A friend forwarded me this YouTube video of an extraterrestrial-looking cloud that was apparently filmed over Moscow late last week.
It sure looks freaky, and many thought it was some sort of UFO.
But I'm betting there's a more natural explanation of this. While not having the data around makes it hard to prove, this looks suspiciously like a harmless "Hole Punch" cloud.
Here is a similar cloud taken locally over Everett's Paine Field by Liem Bahneman:
They are created when a jet flies through a stable layer of super-cooled clouds. While most of the photos I've found on Hole Punch clouds look a little brighter and sunnier than the video from Russia, I'll bet there's something similar going on in the physics behind it.
Here is more information about Hole Punch Clouds: