Downtown Mushroom Kingdom

Downtown Mushroom Kingdom is an anomalous level found in some personalized copies of Super Mario 64. It usually takes the appearance of a dark area with blue mushrooms similar in appearance to those seen in Tall, Tall Mountain, along with various Toads throughout the level. It can be accessed through various means, but it is usually found via a secret entrance in the Castle Grounds.

Geography
Downtown Mushroom Kingdom appears to be a dark, underground area with several blue mushrooms functioning as a light source. There is usually a segment with several houses with Toads. The houses usually cannot be entered, though some have reported being able to enter them. The level usually has a path to somewhere else, such as a landfill. This area's only reported enemies are Goombas, which have reported to have a unique color to them in this level. The Toads also commonly have a slightly different appearance.

Stars
There are very few stars reported to appear in this level, likely due to it's lack of reports and generally small size. The list of reported stars include obtaining it from a Toad, defeating a boss known as the Trashbot, and/or platforming to a certain area where the star is present.

BlackJoystick's Documentation
BlackJoystick has documented Downtown Mushroom Kingdom in their video named Downtown Mushroom Kingdom - Early Level Testing. They begin in Big Boo's Forest as Luigi before going to an entrance to Downtown Mushroom Kingdom and entering. The level begins with a room with several Goombas, before branching off into a path surrounded by a bottomless pit that then leads to the main part of the level.

The main part of the level begins with a gate and a Toad, which allows access to the main part of the level, the town. There are several houses, as well as a path to another area known as Princess Subterranea's Castle Grounds, a building known as the Tiny Tower, and a path to the Junk Passage. The player explores before going to the Junk Passage, using a Vanish Cap to enter another area which has a boss known as the Trashbot. However, the footage cuts out and only audio is heard of the boss fight. The video returns with the player collecting the star. They then appear in Princess Subterranea's Castle Grounds, which is a recolor of the regular Castle Grounds, before talking to a Toad and entering Princess Subterranea's Castle right before the video ends.