Drone albums are difficult to tell apart these days, what with the way sustained chords and stretchy notes have become as common as searing Strat leads and never-ending drum solos — true at least in the experimental rock realm, a place overpopulated with intellectuals who spent the past decade studying nothing but the finer works of Brian Eno and Robert Fripp. Not that there’s anything wrong with that; it’s just that records like Choral, Mountains’ third LP and first full-length for Thrill Jockey, are rare in just how alive they seem to be. In this case, Koen Holtkamp and Brendon Anderegg avoided overdubs and piled on as many as 30 tracks at a time, creating a richly textured tapestry of field recordings, live loops, and melancholic guitar/synth/organ combos — none of which are obvious upon the first or even fifteenth listen. For instance, can you spot the thunderstorm sample beneath the nature doc melodies of “Telescope?” Neither can we; and that, fellow disciples of the drone, is just the beginning.