Thwomp

Thwomps are large floating blue blocks with a face that appear in Mario 64. They attack by falling down to attempt to crush Mario. They can be used as a platform for some stars.

Silent Thwomps
Some reports claim that some Thwomps, found in levels seemingly chosen at random, are silent and do not play their iconic "UNNGH" voice clip. It is unclear whether this is the result of personalization or a glitch preventing their sound effects from playing correctly.