The Rock Candy Mines are a collection of old mining and delivery tunnels that used to transport goods to the Candy Kingdom. In "Varmints," it is revealed that Bonnie and Marceline used to roam around the delivery tunnels and spray paint the walls. Marceline also used this route to get Bonnie out of Cheese Kingdom trade meetings.
It is now abandoned and infested with varmints, which Marceline and Bonnibel tried to kill in the episode "Varmints."
The Rock Candy Mines has long delivery tunnels with broken rail tracks and derailed mine carts. There are rock candy fragments scattered everywhere, and there's also a collapsed bridge. On some of its brown brick walls, graffiti from Marceline and Bonnie can be seen sprayed on.