Four years after their debut album, Boards of Canada once again threw open the gates to their mysterious country compound, revealing a landscape that appeared even vaster and more lysergic, full of dripping melodies, plangent drones, kaleidoscopic structures and sampled birdsong. Their fondness for numerology and synaesthesia -- "Music Is Math," "You Could Feel the Sky" -- suggests an interest in laying bare a hidden world where code and emotion combine in cryptic new forms. Wending between ambient sketches and meditative forays, this shimmering album remains their masterwork.