The Boomerang Pit, the original "Bottomless Pit", is a deep, dark, bottomless abyss in the ground that lies near the Mystery Shack. It may be some kind of wormhole where time is nonexistent, according to Dipper. It first appeared in the titular episode "Bottomless Pit!" of the Disney XD animated series Gravity Falls.
The pit's exterior is a large, round hole in the ground, and there is a sign next to it that says "Bottomless Pit" (which was later knocked over by Stan). Inside, there is virtually no light, and the sides are made of dirt and rock.
Dipper first talks about the pit, when he tells Robbie to jump into it when he was looking for Wendy.
Grunkle Stan takes Dipper, Mabel, and Soos there to dispose of their unwanted items, but the strong wind blows all three of them into it when Stan stubbornly refuses to return to the refuge of the Mystery Shack. Each of them tells a story to pass the time as they fall, and they eventually come right back out the top before Stan falls into it yet again.
Various pits also appear as obstacles in the online game "Fright Night", and can be used in PinesQuest to teleport to different areas in the game.
Mabel shoots the combination to Stan's safe out of Bill's hands with a nyarf gun into one of Stan's memories, and down into the bottomless pit to prevent Gideon from getting into the safe and stealing the deed to the Mystery Shack.
Later, the pit is mentioned in the Mystery Shack's disclaimer.
Wendy mentions the bottomless pit to Dipper, saying she'd throw herself in if he ever stopped being her friend.