I sang briefly in a rock fissure at High Rocks, Tunbridge Wells, Kent, England (High as in Greenland being green, sales pitch for the Romantic period picnic spot for those taking the waters) by the Bell Rock. The rock rings when hit, and echoes over the valley, and so does a contralto voice. I think I surprised someone with the strains of Orpheus lamenting Eurydice - seemed appropriate - issuing from below. The way the sound worked in that space was extraordinary.