A Lull in the Sea

Long ago, humanity lived in the sea. But the humans who longed for the land left the sea behind. They cast aside the special raiment they'd been given by the sea god that let them live in the ocean. The thinking of the sea humans and the land humans became as separate as the places they lived, and as countless time passed, they forgot they were once one people. This will change as the land-dwelling human Tsumugu Kihara befriends four students who live in a village at the bottom of the sea.

Synopsis from Anime News Network