The Astronomy Picture of the Day is of lenticular clouds.
I remember the summer my sister and I visited our dad in Ohio. He was stationed at Wright-Patterson, and one of the times we visited the base was for an air show. There were lenticular clouds on the horizon, and Dad explained that they are often mistaken for flying saucers or space ships. We also saw a stealth plane that trip--there's a good air museum on base.