Hidden Grottos are hidden places where a fixed pool of Pokémon will spawn upon entering the screen.
Once you have entered a Grotto, the Pokémon that is spawned will be "locked" as the only Pokémon you will encounter in that Grotto until Midnight. There is also a chance that no Pokémon spawns and the player will miss an encounter that day unless they reset to a point before they entered the Grotto.
Head to the upper-northwest corner of the city, north of the six soil plots. The HM CUT is required to enter.
The Surf HM is required to enter.
The Grotto is located in the southeastern portion of the map. As the forest can be difficult to navigate through, this may take some time to find.