"Mammatus clouds are most often associated with the anvil cloud and severe thunderstorms. They often extend from the base of a cumulonimbus, but may also be found under altocumulus, altostratus, stratocumulus, and cirrus clouds, as well as volcanic ash clouds. When occurring in cumulonimbus, mammatus are often indicative of a particularly strong storm or perhaps even a tornadic storm." woah! I have never seen these before.
This photo is a great example of the use of shutter speed. Lightning happens in the blink of an eye, and most times if your not already looking at the certain place you'll miss it. The speed of the shutter is why this picture was even able to be captured.