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 New Super Mario Bros., Whirlpools appear only in World 6-5. If Mario gets sucked into one, the player must repeatedly press A to avoid dying. After a while, the Whirlpool will shrink, 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, streching from top to bottom, making it easy for the player to be sucked in.
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 touches one of them, he will be sucked into one, being taken to the bottom of the water, where the player can reach other areas. This is due to the fact that Mario cannot dive beneath the water on his own. However, Mario cannot resurface by touching a Whirlpool.