MAMMATUS clouds are most often associated with the anvil cloud & severe thunderstorms. They often extend from the base of a cumulonimbus but may also be found under altocumulus, altostratus, stratocumulus, & cirrus clouds as well as volcanic ash clouds^
This is an awesome picture. Supercell Thunderstorm by Sean Heavey, nationalgeographic: A supercell thunderstorm rolls across the Montana prairie at sunset. This was a winning shot in the 2010 National Geographic Nature Gallery Photo Contest.