Pokémon S12E31 - Gateway to Ruin!

Ash and friends are on their way to the town where Dawn's next contest will be held. They arrive at Mt. Coronet, the massive mountain that divides the Sinnoh region in half. The mountain also divides the region's pokémon, like Shellos. The Blue and Pink Shellos have ended up on the opposite side of the mountain, and while trying to figure out why, Piplup falls into an artificial tunnel, and when they go to rescue him, they come across something sinister.