One of the rarest cloud types to appear in nature are Mammatus clouds.
Mammatus clouds are an epic cloud formation which forms near heavy lightning storms.
As strange and menacing they may look, the clouds also indicate that the worst of a storm has passed.
Pouches of cool, sinking air cause give the Mammatus clouds their meatball-looking shape.
Check out these incredible cloud formations, including the Mammatus clouds hovering over Mount Rushmore.