Subspace

Subspace, alternatively spelled Sub-space  or Sub-Space,  also known as the Hidden World, and misspelled as Sup-Space on one occasion, is an alternate dimension to Subcon in Super Mario Bros. 2. In Subspace, the terrain is dark and mirrored on a single screen, which the player cannot travel beyond. Subspace can only be accessed by use of the Magical 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, all items found in any available red grass are transformed into coins when the player plucks them, giving the heroes an chance of earning extra lives in the slot machine Bonus Chance at the level's end, though they can only do this for the first two visits (any still-standing grass tufts give unripe vegetables afterwards). Depending on the area, players can also collect cherries, Starmen, and POWs. Cherries and POWs do not vanish upon returning to the main level, allowing one to effectively collect/use them twice.

Within this zone, a few jars contain a Subspace Warp that immediately allows the player to skip to another world, like a Warp Zone. In BS Super Mario USA, entering any jar in Subspace sends the player directly to Wart's chamber.

There are no enemies, but Phanto can follow the player into Subspace if they were holding the key. The player can still die by falling into a pit or drowning in quicksand. However, doing so will allow another chance to gain coins.

In the game's two remakes, the background is the same, but the level terrain is colored-in, rather than black. Additionally, in Super Mario Advance, Yoshi Eggs are hidden here after Wart is defeated.

Trivia

 * The music playing here is an arrangement of the overworld theme from Super Mario Bros.