Mamatus (mammatus) clouds

I was driving home from town last night when I looked up and saw these amazing clouds. Went home and got my camera, and they had changed and looked slightly less apocalyptic by the time I took the picture, but perhaps you get the idea. I emailed Clff Mass, Weather Guy, at the UW to ask what they were, and he said, “Mamatus clouds. Rare here, but we get them.”
You can see a more impressive photo by going to Google images and doing a search on mamatus clouds. It is sometimes spelled mammatus, but same clouds.
My photos were taken just south of Burton, looking east toward the mainland.