Deepest Underground

Deepest Underground is a subarea below Sand Kingdom. There are 2 ways to enter this subarea, by going into the hole under the Inverted Pyramid or by falling into a hole somewhere in the desert. It is the place where the Treasure Ring was kept, until Bowser stole it. The subarea has 2 parts: one where you need to parcour with a slippery icy floor and one where you fight the boss called Knucklotec. After you beat the game and there's world peace, the Treasure Ring will re-appear in the Deepest Underground, on the plateau where it was kept on before it got stolen.