The real world ends beyond a thick, black curtain. On the other side is one of Japan's last remaining hihōkan (sex museum, literally "treasure palace") in the faded resort town of Atami, Shizuoka Prefecture — a strange, dimly lit space of questionable morals and dated fantasies.

After handing over my ticket and entering, my eyes adjust to the dark and slowly focus on a bare-breasted mermaid sitting serenely atop a rock, seductively strumming a harp. But, as I soon discover, this is merely foreplay.

Rarely mentioned in English travel guides, Japan's sex museums are neither easy to access nor actively promoted by most locals. Yet they are interesting attractions that give a rare glimpse into the decadence of 1970's and '80s — an era of incredible economic growth, social change and, it would seem, business-minded perverts.