A forest surrounding Controlei. The path straight to the Beach is marked by glowing stones. Leaving the path and exploring the forest leads to a place that is constantly changing, full of both wild Pokémon and mirages (which you can read about in the Handbook). This is the nearest mystery dungeon to Controlei Town. Despite being completely impossible to map or traverse through without getting lost for a few minutes, it's only an E-ranked place.
