The Great Swamp is the Safari-Zone style area in Pokemon Brilliant Diamond and Shining Pearl. This location is where you can go to find rare wild Pokemon (along with a few other items). You will be able to access the Great Swamp after reaching Pastoria City. It is located in the northern part of the city and will cost you $ 500 to enter.
Before entering the Great Swamp, use the stairs to go up to the second floor of the building and use the telescopes. Each use of one will cost you $ 100, but it will also reveal the locations of some of the Pokemon found throughout the area. If you are looking for a specific Pokemon, we recommend that you take a look at these.
You can find the following Pokemon throughout Great Marsh:
When you enter the Great Swamp, you will be given 30 Safari Balls. Like previous games featuring the Safari Zone, when you encounter a Pokemon, you'll be given the option to throw a Safari Ball, throw lures, throw mud, or run. Once the Safari Balls run out, your time in the Great Marsh will be over.
While inside the Great Marsh, be sure to talk to the NPC to the right of the entrance to get the TM Defog. This TM allows you to clear foggy areas later in the game. You can also use the tram in the center of the Great Swamp for easy travel between the four areas.