Following his discovery by Tim Berne in 2009, Pennsylvania-born pianist Matt Mitchell has been confirming that he's a unique contemporary-music phenomenon in a raft of ways - including his playing with Brooklyn percussionist/composer Kate Gentile, whose inventiveness has also caught the ears of demanding innovators including Dave Douglas and John Zorn. The extraordinary Snark Horse, built around Mitchell's and Gentile's compositions and improv partnership, is a six-disc set of nearly 50 improvisations by various permutations of a 10-piece ensemble, on compositions the pair wrote to only a bar's length, but which improvisers can extensively stretch. Short solo-electronics interludes are interspersed with ensemble recordings of startling variety. Mat Maneri's fragile viola lines glide gracefully on the pair's piano and brushes undercurrents on 'asymptotic rest area' (all the track titles are in lower case, and similarly impenetrable) and take on an asymmetrical grooviness when the trio becomes a quartet with Kim Cass on bass. Versatile saxist Jon Irabagon's soprano is brittle and chirrupy in a purposeful Tim Bernian sextet ('compartments/'s partial') shadowy and dark-toned on tenor in a later slow-drifting seven-piece. Trumpeter Davy Lazar adds squirming top-end trumpet yelps to a hooty, rush-hour-traffic episode with Maneri, Cass, guitarist Brandon Seabrook and the co-leaders, fellow-guitarist Ava Mendoza and tenorist Matt Nelson intertwine on a clanging quartet canter that turns increasingly flinty and free. The full Snark Horsekestra plays two tracks, displaying enthralling collective intuitions in both. Snark Horse is no effortless listen, but it's rammed with sonic surprises, improv ingenuity, rousing avant-swing, sometimes even an ethereal tenderness.