The Mysterious Dimension (謎の空間, Mysterious Space) is a sub-world found in the Dark Realm within Adventure Mode in Super Smash Bros. Ultimate. It is located at the end of the north path, and the boss of this sub-world is Marx. Uniquely, this sub-world appears to be based off of various locations in the real world, as well as locations in games, rather than being based off of any one area in a fictional universe. Objects such as large, green pipes from the Mario universe, torn up roads, chunks of Green Hill Zone grass from the Sonic universe, blossoming cherry trees, a Coliseum from the Fire Emblem universe (or the Roman Colosseum), street lights, a pirate ship, and what is evidently a black hole can all be seen. The sub-world appears to have a theme of chaos, destruction, and disarray, a theme fitting the mischievous Marx. The dimension appears to become more decayed and chaotic the closer the player gets to Marx, culminating in a swirling vortex.
The Spirits found here do not follow any particular theme. Instead, they are found by answering trivia questions about the various game series featured in Ultimate; fighting the Spirit that corresponds to the correct answer opens the path forward, while answering the question correctly the first time will award the player with a chest containing extra snacks or Skill Points. The only punishment for an incorrect answer is having to guess again and not having the chest appear.
The boss of this sub-world is Marx. In order to reveal his location, the player must defeat the Tabuu spirit, which is the correct answer of which Spirit created the Subspace Army and waged war.
Eleven chests can be found in this sub-world. The chest will appear after answering the trivia quiz correctly, unless already answered incorrectly.