Subspace

Subspace is an alternate, but possibly parallel dimension to Subcon that turns everything dark and reverses direction. It can only be accessed by use of the Magic Potions (which create doors leading to the land) in Super Mario Bros. 2. When Mario or his friends enter Subspace, they only have a few seconds to get the Mushroom that may or may not appear; it depends on where in Subspace the hero has entered. In Subspace, Vegetables are transformed into Gold Coins when the player plucks them, giving the heroe's extra chances to earn extra lives in the slot machine bonus game. The music playing in the background is the main theme from Super Mario Bros.. In the GBA remake, Yoshi Eggs could be found here after Wart is defeated.

In Super Smash Bros. Brawl the main antagonists' base is located in an area called Subspace, which they are trying to use to abosrb the whole world, but it isn't the same as the subspace from the Mario games.