From the Super Mario Wiki
A Ground-Pound Switch is a type of switch, first seen in Super Mario Galaxy. To use this item, Mario must Ground Pound it (hence the name). Upon pressing this switch, a variety of things can happen, mainly making Star Bits or Coins appear.
There are also timed Ground-Pound Switches, tree stumps which have similar effects, and grey Ground-Pound Switches that make walls fall down, making other areas accessible.
They are also shown again in Super Mario Galaxy 2, having the same functions like in the first game. There is also a new function for Ground-Pound Switches in this game. If Mario Ground Pounds one of the switches, the screen turns grayscale, and the environment and enemies on the respective planet slow down greatly. This new function appears in the Space Storm Galaxy, Clockwork Ruins Galaxy, and Stone Cyclone Galaxy, and is useful in avoiding stage hazards that would be impossible to dodge otherwise.
Ground-Pound Switches reappear in Super Mario Odyssey. One can be found on top of a skyscraper in the Metro Kingdom. Ground Pounding the switch causes a platform to temporarily appear, allowing access to another building. Four Ground-Pound Switches are also found around the Seaside Kingdom, each one guarded by a cage that can only be opened by collecting the Power Moon next to them. Ground Pounding these switches uncorks the Sparkle Water fountains to allow Mario to fight Mollusque-Lanceur.
According to the brochure for the Lost Kingdom, the symbol on Ground-Pound Switches originated there in ancient times. Some enemies, such as Whomps, Mandibugs, Stairface Ogres, and Astro-Lanceurs have the same symbol on some part of their bodies. Hitting the symbol with a Ground Pound will defeat the creature. Meteorites also share the same symbol.
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