Topping the success of their self-titled 1999 breakthrough was never going to be easy, but nothing about Slipknot ever is. Iowa, named for and inspired by the masked nine-piece’s home state, is alt-metal’s magnum opus: a hyper-aggressive 14-track identity crisis worked out at incredible speed and maximum volume. Enduring live staples like “The Heretic Anthem” and the circle pit stirrer “People = S**t” grind up against harsh but oddly tuneful singles like “Left Behind” and the cataclysmic 15-minute title track. Corey Taylor is one of metal’s rare triple threats—adept at throat-rending growls, whiplash rhymes, and clean crooning—and they’ve never been fiercer or faster than here.