The Secret Aquarium

The Secret Aquarium is one of three secret levels in Super Mario 64, where Mario must collect eight Red Coins in a large, flooded room.

The level itself is fairly easy, as drowning is the only way for Mario to die while in this place. Also, the many Coins in the level will help prevent Mario's health/oxygen from depleting. There is a Red Coin at the bottom of each of the room's four corners, while the other four are floating around the center.

The entrance to the level is in the same room as the entrance to Jolly Roger Bay. If Mario faces away from the painting and looks at the other end of the room, he will find two holes in walls. The hole on Mario's right contains a 1-Up Mushroom, while the left one is the entrance to the level.