The Toad Shrine

The Toad Shrine is supposedly a secret location in Princess Peach's Castle. It is apparently located on the mysterious fourth floor. It appears in most personalized copies of Super Mario 64.

There is a door you can go into in one of the paths that leads to a hallway with the standard red carpet full of Toads in black robes (many have dubbed these guys the "Toad Cult"). Interacting with any of these Toads leads them to say, "Pay tribute to him...". At the end of the hallway is a wide opening with two torches on each side of the opening, leading to another door. Which, when entered, leads to a shrine with a checkerboarded floor and a giant Toad statue surrounded by a pool of water (like the one in the castle courtyard) and 4 torches on each side. Like the statue in the courtyard, a message could be made out on the statue, which some believe says "Our lord shall return."

Speculation
Many believe that this is a cut, unfinished level from the July 29th, 1995 build. It is believed that the statue might represent some otherworldly deity resembling a Toad, as implied by the sign on it. It is also possible that this deity might have played a part in the storyline of the game.