Gravity field

A Gravity Wall is a special type of wall that appears in Super Mario Galaxy and Super Mario Galaxy 2. It has the power to control the direction that Gravity pulls Mario within its expanse. The Gravity Walls contain arrows to show the player the gravity direction, as well colors: blue (arrows pointing down), red (or occasionally pink; arrows pointing up), green (arrows pointing the right), gray, and purple (in Super Mario Galaxy and Super Mario Galaxy 2, respectively, arrows pointing to the left). As soon as Mario or Luigi enters the area with the painted arrows and colored background, either character is immediately pulled towards the object that the arrows are pointing to. The direction that the arrows are pointing on the wall (and therefore the direction of gravity itself) can sometimes be changed if the player finds a Gravity Arrow and Spins it or hits it with a Star Bit. Gravity Walls appear in many levels, the most prominent of which is Bowser's Dark Matter Plant, where they act as the main obstacle of the level. The Rightside Down Galaxy and the Upside Dizzy Galaxy feature Gravity Walls as their main elements. Gravity Walls appear only in 2D sidescroller sections of Super Mario Galaxy and Super Mario Galaxy 2, however.

Trivia

 * Though not a Gravity Wall, the atmospheric background of the Flipsville Galaxy acts somewhat like one, as it features multiple arrows which reflect the current flow of gravity.