Arguably their finest album, this follow-up to Opiate showcases Tool at their best, assisted by clean, crisp production, without the muddiness of Aenima. Edgy guitar riffs are complemented by spitting, heavy bass, especially on "Sober" and "Crawl Away". Lyrically, Tool are at their vitriolic best, targeting religious hypocrisy ("Intolerance," "Sober"; always a popular theme), the loss of innocence and its consequences ("Prison Sex"), and deliberate ignorance ("Swamp Song"). Henry Rollins makes a guest appearance on "Bottom," which, along with "4 Degrees," deals with questions of identity. Undertow is also Tool"s most musically adventurous album, lacking the occasionally numbing sameness of Aenima, and with considerably more sophistication than their previous work.