Java Edition:Invalid DIM Folders
In april fools' Snapshots of Minecraft, it is a common theme to have extra dimensions. After updating out of the april fools' snapshots, the dimension files will still remain in your world folder, making this a file discontinued feature.
Obtaining
20w14∞ Dimensions
In 20w14∞, there were a total of 2,147,483,645 dimensions that could be travelled to by throwing a written book with any text into a nether portal. Each dimension generated its own folder, meaning that there were over 2 billion possible folders (just imagine the size of your world save!). All of the Dimensions followed the same naming convention, being named DIM 2 - 2147483645 (eg. DIM123456789), and being located in the root of your worlds save folder (<Your World/DIM###>. As these are dimensions, each of the folders contain, region files, data folder, entities folder, and poi folder.
23w13a_or_b Moon Dimension
In 23w13a_or_b, the moon dimension was a dimension that you accessed by flying 700[test] blocks into the air. Be careful, because after a while, you will start drowning/suffocating like if you were underwater. There is only one DIM folder for the moon, and the moon is very repetitive, with moon cows, craters, different shaped earths (depending on your enabled vote rules), and the occasional lunar base. The moons DIM folder is called the_moon. Unlike the dimensions from 20w14infinite, the folder for the moon is located in a sub folder. You can access it by going, <Your World>/dimensions/minecraft/the_moon.
24w14potato Potato Dimension
Similar to the Moon Dimension, there is one potato dimension that can be accessed through a potato portal, though unlike the moon, there is beautiful (if you think the poisonous potato theme is beautiful) landscape and a variety of biomes. There is even a colosseum where you can take on the Mega Spud boss and even get to encounter the giant, though be warned, they have AI and can kill you. This biome also has a singular discontinued DIM folder. The Potato Dimensions DIM folder is called potato. Again like the Moon it is located in a sub folder that can be accessed by going, <Your World>/dimensions/minecraft/potato.