Off the western coast of Scotland, where volcanic stone sixty million years old meets the patient labor of Atlantic waves, a sea cave called Fingal's Cave has long offered the world something rarer than scenery: a natural acoustics chamber whose resonance sits at the boundary between geology and music. When the young Romantic composer Felix Mendelssohn visited in the early nineteenth century, the cave's peculiar sound seized him so completely that he left with the seeds of an overture already forming — a work that would carry the cave's voice into concert halls for two centuries. It is a story