The Sand City is a ruined underground city seen in "Red Starved," when Finn, Jake, and Marceline, go to find the Spoon of Prosperity. Most areas in the city are dangerous as the ruined sand structures are unstable, making them prone to collapse.
The city lies within a dark sand cavern located under the Desert Lands. It is entirely composed of crumbling structures and columns made out of sand. Some areas feature the sand corpses of the city's previous inhabitants. In addition, there are smaller caverns located throughout the city. The only known way into the city is through a large hole that leads underground.
Little is known about the origins and the inhabitants of the city. However, it is revealed that almost all of its inhabitants were turned to sand by the Crab Demon and his accomplice, Paris. They did this in an attempt to reanimate the deceased residents and use them to conquer the demon's homeland in the center of the planet. By end of "Red Starved," roughly half of the city was accidentally destroyed by Finn, Jake, and Marceline.