When The Stillness Is Broken is a special request in your notebook. It speaks of a blocked cave in southwest Gatama Atoll. Go to Blue Cliff with a saltwater dolphin partner and head to the side of the drop off. Eventually, you will find what looks to be a patch of out-of-place rubble, which you can get help from your dolphin partner to move out of the way (you cannot move it with your strength alone). This will unlock Silent Cavern, and inside you'll find Starck's Demoiselles (as well as other small fish) and a glow which holds the Gemini Coin.
If the player uses the multisensor to scan the Cavern, they will find a large chest that is revealed to be the unique salvage item the Moon Treasure when appraised.