From Bulbapedia, the community-driven Pokémon encyclopedia.
Marion Town (Japanese: マリオンシティ Marion City) is an anime-exclusive location in the Johto region, which was featured in Celebi and Joy!.
Marion Town was a polluted city prior to Ritchie going back in time
The valley town is home to a stone Celebi Shrine which predates its 50-year-old* Pokémon Center. It is also frequented by the Mythical Pokémon Celebi. Over its history, the area has developed from a small forest village with a few wooden houses, to a larger settlement akin to a city.
At first, the town was very modern, with a lot of high-rise buildings, and described as a polluted urban metropolis. A demolition crew were preparing to demolish the historic Pokémon Center in order to make way for a new office block with a new Pokémon Center at its base.
Ritchie, with Celebi's assistance, went back in time and managed to rescue Nick and his father from their deaths in a rockslide. As a result, Nick was elected as the town's present-day mayor. Marion Town had still developed into a city, though with many older buildings, trees, and a flowing river. The town has now embraced its old traditions and was better connected to nature, even going as far as restoring the historic Pokémon Center.
Pokémon seen in Marion Town