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.