* The Unknown: The very nature of a secret door implies hidden dangers. Bilbo didn't know what lay beyond the door, who might be guarding it, or what traps might be in place. This uncertainty was inherently dangerous.
* The Power of Gollum: Gollum was the guardian of the secret door, and he was a creature driven by hunger, desperation, and a deep, twisted nature. He was unpredictable and could easily turn on Bilbo, especially if he suspected Bilbo was trying to steal his treasure.
* The Potential for Discovery: Bilbo's meddling with the secret door could have easily alerted Gollum to his presence, causing the creature to become hostile and alert others, potentially ruining the entire quest.
* The Riddle Game: To enter the secret passage, Bilbo had to solve Gollum's riddles. Failure would have meant certain death, as Gollum was quite capable of killing anyone who crossed him.
* The Labyrinth: The secret door led into a maze-like system of tunnels, where Bilbo could easily get lost or fall prey to other dangers lurking in the darkness.
So, the danger wasn't just one specific threat, but rather a combination of unknowns, unpredictable enemies, and the potential for failure at every step.