Hidden Grotto

Hidden Grotto (Japanese:  Hidden Hollow) is a feature in that allows the undefined to find  with  or items. The contents of the Hidden Grottoes are random, and if they are empty, for each 256 steps, they generally have a 5% chance to regenerate, usually with different content than what was inside on the previous visit; however, the chance can be increased with the Hidden Grotto Pass Power. Hidden Grottoes will not regenerate unless the items are obtained or the Pokémon are or defeated.

Locations
Twenty Hidden Grottoes are located all around Unova. Some areas have two Hidden Grottoes, while others have only one.

Items
Each Hidden Grotto has the same items available, with the exception of the evolutionary stones. The chance that an item will be spawned when a Grotto is regenerated is 80% (40% for hidden items and 40% for visible items).

Items in each Hidden Grotto
Every Grotto has a chance to spawn the following items at the following rates:

Pokémon
The chance that a Pokémon will be spawned when a Grotto is regenerated is 20%.

Pokémon that appear in Hidden Grottoes have non-standard gender distribution rates. Those species that are genderless, all-male, or all-female are unaffected. Most of the rest will be female 30% of the time, but, , , and will be female 60% of the time, , , and  will be female 10% of the time,  will be female 5% of the time, and in White 2  will never be female.