Loading zone

From SMO.wiki

A loading zone is a type of area object usually found within doorways that transports Mario to a different level if he enters it.

Properties

Loading zones mostly appear solid black or white due to their locations (the loading zones themselves being invisible), and they trigger a screen transition of a spinning hat upon entering them. Most are located within doorways, with some being found at the bottom of wells or other pits in the ground, or at the top of where some beanstalks grow.

While most loading zones transport Mario to a different stage, there are a few in the game that transport Mario to another level that seems disjointed but is in fact located off in the distance within the same stage as the original area. In these cases, the game often unloads objects to maintain the illusion that the levels are unconnected. These loading zones are found in the following locations:

Additionally, the Snowline Circuit entrance room in the Snow Kingdom contains two loading zones that are connected to each other, making these the only loading zones in the game that transport Mario to a different location in the same connected level.

Although loading zones can normally only be accessed from one direction, giving the illusion of a continuous doorway or pit, they can just as well be collided with from the other direction if Mario enters them from out of bounds.