Tool's greatest breakthrough was to introduce dark, vaguely underground metal to the preening pretentiousness of art-rock. Or maybe it was introducing the self-absorbed pretention of art-rock to the wearing grind of post-thrash metal -- the order really doesn't matter. Though Metallica wrote their multi-sectioned, layered songs as if they were composers, they kept their musical attack ferociously at street level. Tool didn't -- they embraced the artsy, faux-bohemian preoccuptations of Jane's Addiction, while they simultaneously paid musical homage to the dark, relentlessly bleak visions of grindcore, death metal and thrash. Even with their post-punk influences, they executed their music with the ponderous, anti-song aesthetic of prog-rock, alternating between long, detailed instrumental interludes and tuneless, psuedo-meaningful lyrical rants in their songs. Tool, however, had a knack for conveying the strangled, oppressive angst that the alternative nation of the early '90s claimed as their own. So, the band was able to slip into the definition of "alternative rock" during the post-Nirvana era, landing a slot on the third Lollapalooza tour in 1993, which helped their debut album Undertow rocket into platinum status. By the time the band delivered their belated followup Aenima in 1996, alternative rock had lost its grip on the mainstream of America, and their audience had shaped up as essentially metal-oriented, which meant that the group and the record didn't capture as big an audience as their first album, despite debuting at number two on the charts.
When their first full-length album was released in 1993 (they released an EP a year earlier), Tool won lots of fans with their grinding, post-Jane's Addiction heavy metal. With their dark, angry lyrics and numbing guitar drilling, they appealed both to metalheads and alternative rock fans. When they landed an opening spot on Lollapalooza, their audience grew by leaps and bounds; the increased exposure helped their debut album, Undertow, go gold. Its 1996 follow-up, Aenima, was also a success. ~ Stephen Thomas Erlewine, All-Music Guide
Back, back I say