Mastodon is an American progressive metal band from Atlanta, Georgia, formed in 1999. The band comprises bassist/vocalist Troy Sanders, guitarist/vocalist Brent Hinds, guitarist Bill Kelliher and drummer/vocalist Brann Dailor. They have released five full-length albums to date, expanding their musical borders from early raw Sludgy riffs to later psychedelic multi-layered songs, yet maintaining progressive style with unusual time signatures and odd instrumentation.