If Mario falls into them, he will immediately lose a life and has restart the level. This can sometimes be avoided by using wall jumps or by rapidly performing the jump kick.
While regular Bottomless Pits are mainly found in Hazy Maze Cave or the tomb beneath Shifting Sand Land, the majority of Bottomless Pits in Super Mario 64 and it's remake are found by leaving the boundaries of various levels by jumping too far or missing a floating platform, so Bottomless Pits are mainly met by sheer accident. Tick Tock Clock may have the most prominent of bottomless pits because there is a bottomless pit that expand the entire bottom of the level.
A special kind of Bottomless Pit is Quicksand, which comes in two variations. While the lighter colored variant has Mario slowly sinking in and can be escaped from by mashing the jump button like in the earlier Super Mario games, the darker colored variant is inescapable and will swallow Mario alive, causing him to stick out his hand and lose a life.
Despite what it might seem like, the lighter colored variant isn't harmless either, as staying in it for too long without moving will cause Mario to drown in it regarless.
While the brighter quicksand is mainly found at the edges of the map and the outdoors areas of Shifting Sand Land, the darker quicksand can mainly be found in the area with the rock bridges, which houses some Tox Boxes.
Unlike the Bottomless Pits, quicksand can be safely croseed by using the Koopa Shell. Surfing into a Bottomless Pit with a Koopa Shell will just call Mario to lose a life as usual.