Metal Mario in the Depths

Metal Mario in the Depths is an unused level found in some personalized copies of Super Mario 64. It is usually found in the Fourth Floor of Princess Peach's Castle, but other methods were also found. It is very similar to to Wing Mario Over the Rainbow and another anomalous level, Vanish Mario in the Pipe Land, being a final test with the powers of the Metal Cap.

Description/Geography
Other than the two main methods of finding the level in Edge of the Mushroom Kingdom or the fourth floor, another method is found by entering a door in a previously empty crevice in the toxic maze in Hazy Maze Cave.

The level itself is said to be somewhat similar to Wing Mario Over the Rainbow, but with different methods of traversing thanks to having different power-ups in use. The level itself starts on a small piece of land right outside an abyss filled with dark water. Touching the water kills the player on contact, requiring the Metal Cap to enter.

Missions:
The most commonly reported missions found in Metal Mario in the Depths are finding 8 Red Coins on platforms, and a similar star finding 8 Red Coins scattered elsewhere, both of which make the star spawn at the bottom. The final most commonly reported mission in this stage is a boss fight with an anomalous boss called the Metallic Monster, commonly reported to be found in Hazy Maze Cave. To defeat the boss, you need to hit it 10 times until it dies. Note that in any of these missions, the Metal Cap has not only a longer lasting time, but also has normal walking speed as if the cap wasn't collected.

Theorizing:
With the existence of both this stage and Vanish Mario in the Pipe Land, many believe that every cap powerup in the game was going to have a second level entirely focused on its use through the level. It is unclear why the two levels were removed with some suggesting that the boss fights encountered by players was supposed to be the main mission for both stages, and that the developers didn't have enough time to complete them.

The reported boss fight with the Metallic Monster is sometimes reported to be glitchy, giving another reason this stage was removed. Touching water that kills you would be somewhat implemented into jungle levels with lakes of poison in future Mario games.