Pile Driver Micro-Goombas are a type of Goomba who hide in blocks and jump around. They were introduced in Super Mario Bros. 3 though have appeared in a variety of video games. Being surrounded by a block, the micro Goomba is invulnerable to fire attacks by either of the Mario brothers. Under the block is a Micro Goomba rather than the more common, larger Goomba. When either Mario or Luigi jumps on top of the block, then the Micro Goomba underneath will be revealed, dramatically reducing its defense.
In the levels they appear in, they often disguise themselves as regular blocks, then jumping when one of the brothers gets near them. In the desert levels, for example, a few of the blocks found on any particular pyramid may in fact be a Microgoomba in disguise.
In New Super Mario Bros., the enemy Blockhopper was very similar to the Pile Driver Micro-Goomba, with minor differences. The Pile Driver Micro-Goombas also appeared in the Super Mario Bros. 3 TV show.