Metal works with two basic sounds: the sound of catastrophe and the sound of triumph. Obviously this is an unfalsifiable theory. But I submit that since the demise of Iron Maiden, catastrophe has been the dominant mode. It just sounds tougher. In the nearly 30 years after Reign in Blood, though, it has also gotten a little tedious.
Maybe that's why Mastodon's Once More 'Round the Sun is so pleasing. It marks the return not only of one of metal's smarter bands, but also of a sound that can be periodically, unabashedly fun. Certainly, Mastodon is capable of screaming anguish, and tracks like "Aunt Lisa" remain firmly grounded in collapse, even with a choral chant at the end. But "Tread Lightly" and "The Motherload" reach for something more optimistic—maybe even bright.
Metal should be fun. No one wants to listen to "Crazy Train" all day, but neither should we wallow in the humorless grind of, say, Cannibal Corpse. Mastodon is near one edge of the sonic palette, and it's not for everyone. Once More proves that they are a valuable corrective to what can be the narrowness of the genre, though. You need some bright colors to render a complete world—even a dark one.