Mammatus clouds are a sign of an extremely unstable atmosphere. They are often seen here on the American Great Plains on the underside of a supercell anvil.

The closer they are to the ground, the more impressive they seem, but they are a treat wherever you might see them. To capture them side-lit by the setting sun is especially wonderful. Nice work!

Four have sold within the last 30 days or so on the U.S. eBay. Average price there $300 USD. I have one on its way that cost me $90 shipped from a seller who is moving to a 645Z. (He also had a FA 200mm macro for $3K USD.)