Seireiko ~Mireille no Namida~

Main character majors in fantasy university at a university in France. His research brings him to a forest lake where he meets beautiful girl Mireille who visited him in his dreams. When he kissed the girl he had no idea he was kissing spirit of the lake. The fairies of the lake dragged him to the bottom of the lake to a new unexplored world.

[From vndbreview]