The Grim Adventures of Billy and Mandy (2001)

Season 4: Episode 7

When Billy finally gets a "secret decoder ring" in his Blasto Bits cereal, he receives a holographic message from a strange creature who speaks about finding the "Secret of the Universe" in an abandoned mine shaft. All the other children of Endsville, also with decoder rings, race to the mine to be the first one to get the Secret. The entire effort is revealed to be a trap laid by Lubbermouth, a worm-like creature from Grim's past, who wishes to free herself from the curse of guarding the Secret. When Mindy looks down at the pit, she becomes the new guardian of the Secret and has to stay there forever.