Silent Cavern is a small, somewhat well-hidden cave in Endless Ocean: Blue World, carved into Gatama Atoll's Blue Cliff.  

Silent Cavern

The Entrance to the Cavern.

It is first mentioned in a special request in the player's notebook called "When the Stillness is Broken", and to complete this request (and gain entrance to the Cavern), the player must get one of their dolphin partners to move the rocks that block the entrance to this small cave. However, this dolphin partner must not be the Amazon River Dolphin (if the player has already obtained that particular dolphin at that point, which is unlikely as this special request appears rather early and the Amazon River Dolphin is a late-to-post game partner), because that species lives in freshwater when in the wild. This means that it cannot gather enough strength to help the player move the rocks, as it is outside of its natural habitat, and the saltwater may be making it slightly sick. 

Inside of the dead-end Cavern, the player will find the Moon Treasure (once they use the Mulitsensor) and the Gemini Coin, as well as a shoal or two of small fish (such as King Angelfish).