Thwomps are animate cinder-blocks found mostly in the Mushroom Kingdom. They are spike-encrusted living stones that are usually rectangular in shape and made their first appearance in the game Super Mario Bros. 3.
They typically attempt to fall on and flatten or squash whoever passes below them, although some move horizontally or diagonally as well. They can only be defeated by being touched by Mario when he is powered up with a star, using hammers as Hammer Mario, turning into Statue Mario, using a Gold Flower, or being touched when Mario is giant. It is suggested in Mario & Luigi: Partners in Time that Thwomps may in fact be born from volcanic activity.