tsuioku no chouseki

A young man named Nagi lives in the deep sea, and a woman named Kurage lives on the surface. The two, who reside in worlds that never intersect, share memories through the phases of the moon and are drawn to each other's souls without ever seeing each other's faces. However, their souls are descended from a single mermaid, and if they were to meet, the world would collapse. Accepting their fate of never being able to meet, they offer glassworks-crystals of love-to the sea as their final memory. This is the eternal tragic love song played by two people divided by the world.