The cave of sadness.
The Cave of Sadness is a cave in the Star World, first appearing in MarioMario54321's Super Mario 64 Bloopers movie The Cosmic Crystals. It is described as a dangerous place due to its sadness-indulging powers, capable of making even the happiest explorers leave the cave tearing up. Appeareance-wise, there are sad faces everywhere, seeming to be covering every corner of the cave, along with obstacles that reflect its themes of sadness (such as the Pit of Pity and the Boo-Hoo Boulders), and even having demoralizing messages written within its walls. The only artifacts that bring happiness in the cave are the Cosmic Crystals, which are found at the very end of the cave, in a secret room.
Later on, in the 5th episode of The Cosmic Crystals, it is revealed the cave was created by Stario, in search of a worthy hero that was capable of braving through the cave's sadness-indulging powers to collect the Cosmic Crystals, in order to defeat Mastar.
However, in the 3rd episode of The Moon World, it was revealed that the joy in the Cosmic Crystals was fake and was inserted by Optimismo to strengthen himself. The cave also has a dinosaur named Teary.
It turned into the Cave of Happiness in The Cosmic Crystals III: The End of the Star World.