The Box of Secrecy is the site of emergency G.R.O.S.S. meetings when the tree fort is inaccessible or inconvenient to reach. It is a cardboard box with "G.R.O.S.S." written on the side. It was used only once, in the last G.R.O.S.S. storyline, when Susie Derkins stayed at Calvin's house. The Box is so secretive that air holes are considered a breach in security. The "Box of Secrecy" is most likely a parody of the "Cone of Silence" from Get Smart.