The mother of all clouds

There were storms all around us this afternoon but nothing actually happened here apart from a few rumbles of thunder and about ten large drops of rain. But the clouds looked dramatic, and I was able to photograph this as I drove out to book club. (OK, I stopped the car to take the photo.) It's a mamma cloud I believe, breast-like formations (or udders of cloud as Pretor-Pinney describes them in his book The Cloudspotter's Guide) that can hang beneath a Cumulonimbus, and indicate high instability around the top of the cloud.
PS I pinched my heading from Pretor-Pinney as well.