I don't know the history exactly, but at the end of the 20th century, a series of recordings (or, more accurately, collections of recordings) was made available in the Western world under the title Le Mystere des Voix Bulgares. The female choral tradition in the Balkans generally, and Bulgaria specifically, has deep roots and is more on the order of classical than folk music: these are not peasant women overheard whilst laboring out in the fields. (Well, for all I know, they may be peasant women, but they are also unquestionably artists of advanced and trained vocal talent, performing a repertoire of challenging and jaw-dropping beauty.)
One hopes they have been compensated adequately, but again, the story of how these recordings got to us is even more a mystery to me than where these ethereal sounds come from.
My favorite of the editions with which I am familiar is Vol. 3, so I'll start there as a recommendation, but to uninitiated ears I would imagine they will all sound more or less the same at first. Complicated time signatures, dissonant harmonies--and no one's going to mistake these choral women for Allison Krauss. Balkan singing is forceful and bracing and dares you to be ready for it. Better be standing with your knees bent or it may knock you over. I know these insistently gorgeous sounds knock me out every time.