Cave Island

Cave Island is a tiny Second Sea island worth visiting for its chests and as a safe base for sea events. It lies in a remote area to the east of the Kingdom of Rose docks, and the wiki says it has no official name. Three chests are inside: a silver one on one side of the large main cave, a gold one on the grassy top, and a diamond one behind a partial wall in the upper room. That upper room is hidden and reached through a small square opening in the ceiling, so look carefully near a wall. It is also one of five spawn locations for the blue flower, which only appears at night, on the mint-green grassy carpet. The island has no music of its own, so the regular sea music plays, and you cannot reach it with Portal's World Warp. Because the game does not count it as a real island, a flying boat can land on its floor. That makes it a good place to hunt sea events: Sharks and Piranhas cannot get inside, so your boat keeps its health. Watch out for Ship Raid cannons, which pass through the walls. A Poneglyph used to be here until Update 13 removed it.