The Secret Aquarium

The Secret Aquarium is one of the secret courses in Super Mario 64 and Super Mario 64 DS. The entrance to the course is to the right of the painting into Jolly Roger Bay, through a small hole up above on the wall.

Geography
The Secret Aquarium is a large fish tank filled with water. There is no place for the player to surface for air, so they will have to collect coins in order to avoid drowning. There are 40 Yellow Coins in rings with Red Coins in the center of the rings and on the room's floor. The only Power Star to collect is the one that appears when the player collects all eight of the Red Coins around the aquarium.

Aquarium Limbo
Main Article: Aquarium Limbo

In some personalized copies, the Aquarium Limbo can be found. The Aquarium Limbo has some differences to the Secret Aquarium like the Big Boo's Haunt background, darker lighting and the lack of both yellow and red coins, so the player has no choice but to let Mario drown and die, if he dies, the game crashes due to the Textureless Mario Anomaly occuring in this stage.

Alternate Entrance
According to some reports, the Secret Aquarium can be accessed in the second hole in the wall in the room leading to Jolly Roger Bay in Princess Peach's Castle, or by somehow entering one of the small aquariums seen in the same room, either by walking into it, or punching the aquarium, causing the glass to break and reveal the warp.