The Mountain of Matthew is a region in the Land of Ooo. The mountains can be seen with a telescope from afar. The mountain was explored by Finn and Lemongrab as they struggled over spiritual obstacles inside in "The Mountain." Jake was not allowed in the mountain. The mountain also has a section where Matthew (the ruler) was living. It is accessible once all of the obstacles are complete.
The inside of the mountain is a huge cave with brown ragged rocks and dripping water. The mountain's interior is also shown to have a path leading to three glass panels where a being who explores the cave must choose between three options to continue their journey (shown in "The Mountain"). The choices are related to the being's mind and then used as an obstacle. Once the spiritual obstacle is overcome, the being may enter a section where they would come across Matthew (the ruler). However, due to the events in "The Mountain," Matthew is deceased and the current situation in this mountain is unknown.
Only a being with troubles or something causing them discomfort mentally may enter the mountain. A being exploring the mountain will come across three glass panels on which three choices will be displayed to decide their direction as they continue their journey. The choices are related to the being's mind and then used as an obstacle. These obstacles are to de-stress a being's problems and use their troubling factors in their mind to conquer these problems. The obstacles were seen in "The Mountain" as Finn and Lemongrab were exploring the mountain and their troubled thoughts were used as an obstacle in the mountain.