Stratocumulus Twirl Clouds in Grytviken, South Georgia Island

Strange Altocumulus twirl clouds (Altocumulus lenticularis) in the sky above Grytviken, South Georgia island, at the cold edge of Southern Atlantic Ocean.

Credit: Nasa

This form of altocumulus lenticularis (lenticular cloud) can resemble flying saucers and may occasionally be mistaken for "unidentified flying objects". 

This type of cloud formation is normally formed by uplift of cool air and is usually associated with mountains.