Much like other Pokémon preserves in Kanto and Hoenn, the Great Marsh features a Safari Game in which Trainers pay 500 for 30 Safari Balls and a limited number of steps (500) to catch all the Pokémon they can.
The mechanics are similar to other Safari Zones: when a wild Pokémon appears, the player may not send out any Pokémon to battle it, but must instead try to catch the Pokémon without harming it. The wild Pokémon may choose to flee the battle at any time, thus, catching a Pokémon in the Great Marsh requires mainly luck.
There are four options shown in the battle screen:
Throw a Safari Ball - Throws a Safari Ball in possibility of capture. It is much the same as throwing a Poké Ball in an average Pokémon battle.
Throw bait - Throwing Bait makes a Pokémon harder to catch, but makes it less likely to flee from battle.
Throw mud - Throwing Mud does the reverse of throwing Bait, making a Pokémon more likely to run but easier to catch.
Run from battle - Like in an average Pokémon battle, running away from a Pokémon will cause the battle to end.
The area was redesigned in Pokémon Platinum to feature slightly upgraded graphics that allow the player to distinguish between deep and shallow mud. The park is still divided into six areas, but the individual areas have slightly different layouts.
Using the binoculars to spot Pokémon
Some Pokémon will not always appear in the Marsh. Each day, the Pokémon will change; therefore, to get a new set, players must wait a day. These Pokémon will appear randomly in one of the six areas; some may not appear for a while, while others may appear several days in a row or in multiple areas on the same day. Some may not appear until after getting the National Pokédex. The only way to tell which changing Pokémon is available in the areas for the day is to use the lookout upstairs from the entrance. Record mixing also changes the changing Pokémon of the day in the Great Marsh so that all players mixing records have the same Pokémon on the same day.
In order to find out what Pokémon are going to be in each area daily, the player is able to walk up to the second floor in the lobby, and use the binoculars and look out onto the marsh. For 100 players can get a brief glimpse of the Great Marsh showing certain Pokémon in certain areas of the marsh. In each slide, a part of the park will appear with a Pokémon in the center of the slide that can be found in that area.
As with previous Safari Zones, Pokémon are made easier and harder to catch by modifying their catch rates. An escape rate is also given for each Pokémon that appear in the Great Marsh. The escape rates for Pokémon are the same for all Generation IV games, except that those who do not appear in the Great Marsh or the Johto Safari Zone do not have their escape rates defined in the relevant games.
At the start of an encounter, both rates are set to their species defaults. Throwing Bait will lower the escape rate by one stage, but will also have a 90% chance of lowering the catch rate by one stage. Conversely, throwing Mud will raise the catch rate by one stage, but will also have a 90% chance of raising the escape rate by one stage. Stages for catch rates and escape rates work identically to stat stages: they may not be raised above +6 or below -6, where 0 is the starting stage. The stage multipliers applied to the catch rates and escape rates are also identical to that of stat stages.
At the end of each turn, a random number between 0 and 255 is generated, and if it is less than the modified escape rate, the Pokémon escapes.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
The "rates" below are the chances for that Pokémon to be selected to appear in an area. All of these Pokémon will appear with a 10% encounter rate within the Great Marsh, or a 10% greater rate if they were already present. In Diamond and Pearl, the changing Pokémon's encounter rate is compensated (so that all encounter rates sum to 100%) by lowering the encounter rate of Marill and Quagsire in the same area (as seen above) by 5% each, while in Platinum, it is compensated by lowering the encounter rate of Wooper by 10%. The way this works in the games, this means that if Wooper is selected as the changing Pokémon in Platinum, there is no change to the tables shown above.
As a boy, James was exploring the Great Marsh when he found a Carnivine stuck in the mud, struggling to get free. James pulled it out and they became friends ever since. During Team Rocket's journey through Sinnoh, Carnivine traveled with James.
In A Skuffle with Skorupi, Diamond, Pearl and Platinum came here after Crasher Wake's assistant recommended Platinum to get a Grass-type Pokémon before challenging the Gym Leader. Platinum tried to catch various Pokémon, like Budew and Skorupi, with no success. At some point, the group got stuck in the mud. While they tried to struggle themselves free, a wild Carnivine attacked Chatler and Lax, who fought it and won. Chimler saved Platinum from drowning in the mud, after which the group left the Great Marsh empty handed.
In Diamond and Pearl, a man in the Great Marsh incorrectly states that both the Bait and the Mud make Pokémon more likely to run but easier to catch. In Platinum however, the man states that the Bait will make the Pokémon easier to catch and more likely to flee, and the Mud will make it harder to catch but less likely to flee.
In Platinum, if the player encounters a Pokémon when they would normally would "pop" out of deep mud, a glitch exists where upon exiting the battle, it will seem that the player is still "stuck", despite being able to move around normally. The effects of this glitch are most obvious when the player attempts to move to a tile with dry land after performing the glitch.