There are hidden areas spread through Pokémon Scarlet and Violet The Teal Mask’s new map but the expansion doesn’t really add too many secrets that are really worth finding; expect, as it turns out, for players who really like Magikarp and want to capture the strongest one possible.
Off in an isolated corner of Fellhorn Gorge, there is a not-quite pond area between two very small strips of land. You likely would never stop by here unless you really planned to explore the entire map, though that may be a mistake for some players.
Within this small area, there sit some very high leveled Water and Flying-type Pokémon. Most of them range between level 60 and 75, and then there’s one clear outlier you could be lucky enough to spot on your first visit.
After seeing a player on Reddit post about finding a level 100 Shiny Magikarp in the area, I went and visited it for myself. While I didn’t find a Shiny, I did encounter a level 100 Magikarp in that exact same spot—though mine was flopping around on the small strip of land separating it from the gorge’s main water source.
After catching the mighty beast, I scanned the surrounding area to see if I could find more maxed-out Magikarp. Unfortunately, all I found was karps ranging from level 50 to 70.
I left the spot a few times to forcibly respawn the encounters but, outside of some fairly high-leveled Gyarados and Drednaw, I didn’t see anything else worthwhile. This means I wasn’t able to confirm that this level 100 Magikarp is a guaranteed encounter or if it will respawn occasionally.
It did seem like a hotbed for Magikarp spawns, however, so it might be a great place to Shiny hunt if you can get the right sandwich powers rolling for you.
This location is also one of the few spots you can grab a Prism Scale to evolve your Feebas into a Milotic too, if you were having trouble there.
