These are mammatus clouds. The name means “breast clouds,” for obvious reasons. They form when the underside of a thundercloud, laden with ice crystals, cools the air below and condenses the moisture in it. They almost always hang down from the bottom of storm clouds or rain clouds, especially at the edges.
Because people saw these smooth clouds, and then saw the towering thunderclouds behind them, and assumed these clouds were signs of terrible storms to come. Actually, meteorologists have found that they tend to form only when the storm is clearing up. So rejoice when you see boobs in the sky. Things are getting better. Via We’ve finally figured out what these strange breast-shaped clouds mean!.