Mt. Moon was a dungeon from PMU 6 found in the Archford Region. The first portion of Mt. Moon was the ground - middle height level of the dungeon. Once you got past the Mini-Boss room and the room with the Dragonairs and Monfernos, the player would reach an overworld snowy mountain portion of the dungeon (Yes, this was technically still part of the dungeon).
There was an Inn found near the cave entrance to the peak of the mountain; this Inn sold a highly used item in PMU 6 called a Poffin (Specifically the attack raising varient, this was removed from the Inn several months later considering it was quite broken for such a low price.)
The cave reaching the peak of the mountain was a only a few maps, the last two floors before the Jirachi (Mirage) room were packed with Piloswines. This was a very good place to grind for both EXP and Money the Piloswines had a short respawn time and they had about a 30% drop rate of Pearls (Which was good vender trash that provided roughly 5k/pearl)
The final boss fight was on the peak of the mountain, there was a night skybox in the background. Once Jirachi was defeated, you would be able to go to the right side of the boss room, where you were able to jump from the peak of the mountain and fall into the Squirtle Pond that was next to the entrance of Mt. Moon.
Below are the PMU 6 bosses.
Mini-Boss: Umbreon and Espeon (Names: Kage and Hikari)
Mini-Boss: Jirachi (Mirage)
Set Level? No