Not exactly underground but at this point certainly off the beaten path if only because it's out of print.... Jon Anderson (of YES, not the country singer) "3 Ships" 1985, out of print but can be found if you look.
My two favorite remain John Prine and Leon Redbone. After you hear Redbone do White Christmas, Bing sounds hyper. His rendition of Christmas Island is great too. Prine not only sings, but tells great stories.