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.
Eleven chests can be found in this sub-world. The chest will appear after answering the trivia quiz correctly, unless already answered incorrectly.