From Bulbapedia, the community-driven Pokémon encyclopedia.
Whiscash Pond (Japanese: ナマズンのいけ Namazun Pond) is an area in Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team, to the north of Pokémon Square. Its main inhabitant is Whiscash, who tells stories to the player. He is the first to tell the Pokémon of the Ninetales legend. Once the player has stopped a meteor from destroying the world, an area called Luminous Cave appears in the center of the pond, where Pokémon can evolve if they are a high enough level, have an evolution stone in their possession, if their IQ is high enough, etc.
In the anime
Whiscash Pond in the anime
Whiscash Pond appeared in the anime special Pokémon Mystery Dungeon: Team Go-Getters Out of the Gate!. The newly formed Rescue Team Team Go-Getters visited the pond to ask Whiscash for help. They found a worried Pichu there, who had told Whiscash that his older brother Pikachu had gone missing after visiting Sinister Cave. Whiscash agreed that Team Go-Getters and Team Meanies should go and find Pikachu.