Unagi Caves

Unagi caves are a cut area in Super Mario 64, in the level Jolly Rodger Bay. Seeing it is a sign of personalization of the game cartridge.

Area
In Jolly Rodger Bay, there is an enemy known as Unagis, which are giant eels. In most versions of the game, there is only one. This one will come out of a tunnel with a star on its end that the player can collect. It should be noted that the player can not enter the tunnel most of the time.

However, if the personalization level is high enough, the player will be able to explore the tunnel. The tunnel leads to a massive labyrinth of underwater caves with multiple Unagis. The Unagis here are very aggressive and will try to chase Mario around the area. Also, there are some areas above water, where the player can rest. Lastly, there is a star in a cage in this area, but it is unknown how to obtain it.

Theories
A popular theory is that the AI will make this area when it thinks the player wants a challenge.