Gaia Falls Islet

Sail east of Yamata to find a beach and a cave (ガイアフォールの洞窟, Gaia Fall's Cave).

Walkthrough
Go down the stairs here. Unfortunately, when going down to the bottom, the waterfall will make it impossible to continue to the left, so aim for the southern exit. You'll be on the world map, but just go into the next cave to come back.

In here, move up and Grip yourself over to the left and find the Umbra Cloak. Now go back across the gap. Make your way up the water on this side by using the poles and grabbing from one to the next. Go up and use the big rock to edge yourself to the spire. Just take your time here and don't fall to the lower rock. On the second grab, run to the left to reach dry ground. Go up and cast Growth on the vine to reach the chest, but it's a Mimic. Defeat it to gain an Apple. Now go up the stairs and walk into the waterfall to find the final Mercury Djinni, Shell.

Collectibles

 * Umbra Cloak
 * Apple (from Mimic)
 * Mercury Djinni Shell