The Royal Order of the Holy Mackerel

The Royal Order of the Holy Mackerel is a secret society. It is mentioned in Rumble's Revenge in a cryptogram that says, "There is a secret society in Gravity Falls." Stan is probably a member, because a cryptogram in Made Me Realize says, "Stan is not what he seems." Soos may be a member too, due to another cryptogram in Rumble's Revenge saying, "The handyman knows more than you think." Bill may also be involved.

Trivia

 * It's name was proven in Mystery Shack Mystery, because scrolling over a much smaller picture of it's symbol on the wall will make the caption "The Royal Order of the Holy Mackerel" appear.
 * It is possible the secret passage behind the vending machine in the Mystery Shack leads to it's headquarters.