Head east from Mabe Village into Ukuku Prairie. This expansive area contains many secrets, including your first Secret Seashells. Look under bushes, in suspicious patches of grass, and by lifting rocks. The Seashell Mansion is located in the northern part of Ukuku Prairie. While you can't get many rewards yet, it's good to know its location. Continue exploring to find the entrance to Bottle Grotto, which is north of the Mysterious Forest.
