Heart Pillar

From the Super Mario Wiki

A Heart Pillar.

The Heart Pillars are tall pillars scattered around Flipside and Flopside that are used to house the Pure Hearts in Super Paper Mario. Once the four heroes place a Pure Heart into a Heart Pillar, a new doorway to another dimension becomes available.

There are eight heart pillars in the game, and they are mostly found in the outskirts of Flipside and Flopside (though the game's first Heart Pillar is found on floor one of Flipside). The last Heart Pillar in the game is invisible until the four heroes get the Pure Heart from The Overthere.