Whirlpool
A Whirlpool is a spinning column of water that appears in some Mario games. Usually, its purpose is to suck the player down into it, causing damage or even losing a life.
In Super Mario RPG: Legend of the Seven Stars, Whirlpools appear in almost every underwater area. However, these Whirlpools provide an actual purpose. When Mario is sucked by one, the plumber is taken to the bottom of the water, where the player can reach other areas. This is due to the fact that Mario cannot dive underwater on his own. However, Mario cannot resurface by touching a Whirlpool.
A Whirlpool appeared in Super Mario 64 and its remake, in Dire Dire Rocks. It appeared in the middle of the first area, and if Mario or others go near it, they could possibly be sucked in.
In New Super Mario Bros., Whirlpools appear only in World 6-5. If Mario gets trapped into one, the player must repeatedly press A button to avoid being sucked down. After a while, the Whirlpool will dissapear, and Mario can pass by. However, one Whirlpool allows Mario access to a Star Coin.
In Super Mario Galaxy, Whirlpools appear underwater in the Sea Slide Galaxy, where Mario must avoid them when racing Guppy or the Penguins, otherwise taking damage and falling behind in the race. These whirlpools are together in a small area, stretching from top to bottom, making it easy for the player to be sucked in.