Brian's answer is correct. 'In' is a two way preposition. Here a link to these prepositions, which can take either the dative or accusative: https://germanwithlaura.com/two-way-prepositions/ Ich bin im Supermarkt. Where? Asking for a location. Ich gehe in den Supermarkt. Where to? Direction and movement.

The preposition “in” takes the accusative (den Supermarkt) when referring to a destination or movement “into” a structure or location. It takes the dative when referring to movement within a location where the person already is (im Meer schwimmen).