Drip Drip Cave

Drip Drip Cave is a secret course found in some personalized copies of Super Mario 64. The level is a large, dark cave filled with water, with very few spaces to get air from. The level usually gets progressively darker the deeper the player swims.

The level itself is usually accessed by glitching through the panel blocking entry to the passage to the Castle Grounds in Dire, Dire Docks. It will usually not be able to be accessed upon collecting Boarding Bowser's Sub mission, which will then open the passage. However, some people have accessed Drip Drip Cave through the open passage, though this is uncommon.

Geography
Drip Drip Cave is described as a narrow and linear cavern, with a top area reportedly consisting of several Goombas and coins that then leads into the water. There is also usually a Metal Cap at the top used to reach the bottom of the level quicker.

The flooded area is commonly a drop straight down, progressively getting darker as the player swims down. People have reported Boos, Unagis and Bubs throughout this area, becoming more common the deeper down the player goes. There are usually small areas in the walls of the level where the player can regenerate air and health.

The bottom of the level has been reported to have a sunken ship similar to the one seen in Jolly Roger Bay, that is surrounded by Boos. This area usually contains one of the course's star.

Stars
This level has been reported to have very few available stars. The stars that are available have reportedly consisted of a 100 coin star, a red coin star, a star at the ghost ship, a star for fighting a Big Boo at the ghost ship (usually using the Metal Cap), and a star connected to a Unagi's tail similarly to Jolly Roger Bay. Strangely, one report states they were able to find 2 secrets, but not enough to obtain a 5 secrets star.

Theorizing
Due to Drip Drip Cave usually appearing relatively complete, it has been theorized that the course was cut late into development. The reason it was cut isn't known, though it could potentially due to the level's difficulty and similarity to Jolly Roger Bay and Dire, Dire Docks.

Since the level can be accessed from glitching into the passage door, therefore entering it prematurely, it can be presumed that this was the original intention to enter this level, as opposed to the warp taking the player to the moat of the Mushroom Castle. This warp remaining here whilst the door is closed might be a remnant of this intention, left unchanged as developers wouldn't expect any player to clip through the door and into the warp.

Alternatively, the level has been speculated to be made by the Personalization A.I. through Dynamic Level Arrangement due to it's recycling of assets, though this is unconfirmed for now, as others have said the assets being recycled could have also been done by developers.