Pokémon Diamond and Pearl/Pastoria Great Marsh

The Pastoria Great Marsh contains many Pokémon that are unobtainable elsewhere at this point in the game (assuming you are following the walkthrough). The only this "needed" at this point is the Defog Hidden Machine which is obtainable by talking to the man to teh right of teh enterance, so pay that 500 yen and get it if you don't want to be stumbling around in the fog in 2 parts of the game.