The Old Bell is a book in the library of Vasquez Castle.
Scooby-Doo, Where Are You!
While stranded inside Vasquez Castle, the gang find it to be haunted by the Phantom. While investigating, they find a key with a message at the top of the handle saying "Second Passage of The Old Bell". They deduced that it meant the second passage of a book, and went to the library to look for it. They found the book with the word inside underlined, instructing them to go to the basement of the castle.
- In trying to give a hint in the clue, the key probably meant to find a passage in the second page of the book. Either that or there was bad communication between the writers and animators. Not only that, they're also half way through the book.