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 a rare meteorological phenomenon called a ‘SKY PUNCH’. When people see these,they think it’s the end of the world. Ice crystals form above the high-altitude cirro-cumulo-stratus clouds,then fall downward, punching a hole in the cloudcover.