The Secret Aquarium is a secret location in Princess Peach's Castle in the games Super Mario 64 and Super Mario 64 DS and this level can be found underwater. Mario must get all eight Red Coins in the water to get a secret Star. This area is one of the few areas that cannot be seen by going through a painting. The red coins are found in the bottom corners and halfway up the wall between each of corners.
The Secret Aquarium can be found in the room of the castle with the Jolly Roger Bay painting in it. When Mario enters, he must walk forward a little bit, then turn around. Mario will see two black squares on the top right corner of the wall and the top left corner. One of these has a 1-up Mushroom in it, the other has the Secret Aquarium. There are bubbles that give Mario more air in the depths of the water, and they are vital to his success of getting the secret star.