What Is The "Mystery Terrain" & "Thick Grass?"?

https://www.dragonflycave.com/calculators/gen-v-catch-rate

https://www.dragonflycave.com/calculators/gen-vi-vii-catch-rate

Whatever these things are, they seem to lower the catch rate of a Pokemon by quite a bit. I tried looking it up, but I found nothing on either. Can anyone explain these?

Pleaselog inorregisterto add a comment.

Might as well take this off the unanswered list.

Thick GrassThick Grass (also called Double or Dark Grass) is Grass in the overworld that is darker in appearance. You can encounter Double Wild battles here. Pokémon in this area are usually higher level than other Pokémon on the route. Your success in capturing Pokémon in this area depends on how many Pokémon you have registered in your Pokédex.

Mystery TerrainI cannot find much information on this. The information from the sources provided states the same thing as Thick Grass: The less Pokémon you have registered in your Pokédex, the harder it is to catch Pokémon in Mystery Terrain.

So, all in all, in the mystery terrain, it is extremely difficult to catch Pokémon at the beginning of the game, when you’ve caught few Pokémon, and it only becomes as easy as normal when you have more than 600 Pokémon caught.

I recommend giving the sources a good read, they’re pretty interesting.