In Game Description
"This area is found in the southwestern part of Gatama Atoll, where the seabed drops off sharply and plunges into the depths of the ocean. The cliff wall itself is speckled with a wide range of corals."
At coordinates A-7, at a depth of roughly 20 metres down (About 60 ft), there is a large, protruding boulder. If you get a saltwater dolphin partner to help you push the rock, you will open up Silent Cavern, thus completing the quest "When the Stillness is Broken" in your notebook. Inside is a tiny tunnel filled with a shoal of Starck's Demoiselles, and you will have to use your multisensor to find a treasure at the very back of the cavern.