The Bottomless pit is a portal to The Nightmare Realm. In the publication of Journal 3, there is a code that was written by Bill whenever Ford was asleep. On the Bottomless Pit page, the code reads: "Some of this junk ended up in my dimension," referring to the items that Ford threw in as a test. The reason Dipper, Mabel, etc. didn't end up there, is because weird objects are attracted to the town of Gravity Falls, meaning, since they were all drawn to the city in the first place, they would always be attracted to it, so the kind of would just fall up.