Princess Peach's Castle/Below the Castle

Below the Castle, also known as the Castle Grounds Cavern, is the name given to an anomalous area found in some personalized copies of Super Mario 64. It is described as a cavern of varying size located underneath the Castle Grounds of Princess Peach's Castle. It has also been described as being very simplistic, implying it is created through Dynamic Level Arrangement or was scrapped early in development. It has been said to be accessed through a hidden warp somewhere in the Castle Grounds, as well as appearing in place of Vanish Cap Under the Moat.

Description
Below the Castle is described as a cavernous area serving as an intersection to various different levels or areas, though can serve as it's own smaller level, usually consisting of platforming or collecting red coins, secrets, or anomalous collectibles such as Silver Stars to unlock the level's star(s). However, this is a more uncommon manifestation of Below the Castle.

The main manifestation of Below the Castle is simply described as a large, central room with several hallways leading to levels as well as the entrance to the level, which can appear as a hole in the ceiling in the center of the room or one of the hallways. Some of the hallways simply lead straight into the level, or have a hole in the ground leading to it, a hole in the ceiling that the player has to jump to, or several other variations.

Titanic Tunnel
Titanic Tunnel is the level most commonly reported alongside Below the Castle. It is described as a large cavern similar to Hazy Maze Cave in design filled with several crystals similarly to Cavern of the Metal Cap that can reportedly be shattered and contain several items used to progress through the level. The level usually has the same stars as Hazy Maze Cave, though some other basic stars have been reported such as a 5 secrets star.

Scuttlebug Cavern
Main Article: Scuttlebug Cavern

Scuttlebug Cavern is a cavern level filled with the titular Scuttlebugs. Despite usually being separate from Below the Castle, several have reported that an alternate entrance to Scuttlebug Cavern containing 1-3 stars in the level can be found in Below the Castle, or that the level can only be accessed through Below the Castle if there is only one entrance.

Report 1
This video by First Name Last Name documents Below the Castle as well as an alternate version of the Scuttlebug Cavern. It begins in the Castle Grounds as the player, playing as Luigi, swims into the filled moat and into the Vanish Cap Under the Moat entrance, notably lacking the grill usually blocking access to the level until the moat is drained. Instead of leading to Vanish Cap Under the Moat, it instead warps the player to Below the Castle. It takes the appearance of a central room and 4 different hallways, all made of rock. The player begins in one hallway and looks at the other hallways before going towards one hallway with a hole in the floor before the footage cuts out and the player enters Scuttlebug Cavern, collects a star, and the video ends.