Of all rock & roll's myriad forms, heavy metal is the most extreme in terms of volume, machismo, and theatricality. There are numerous stylistic variations on heavy metal's core sound, but they're all tied together by a reliance on loud, distorted guitars (usually playing repeated riffs) and simple, pounding rhythms. Heavy metal has been controversial nearly throughout its existence -- critics traditionally dismissed the music as riddled with over-the-top adolescent theatrics, and conservative groups have often protested what they perceive as evil lyrical content. Still, despite -- or perhaps because of -- those difficulties, heavy metal has become one of the most consistently popular forms of rock music ever created, able to adapt to the times yet keep its core appeal intact. For all its status as America's rebellion soundtrack of choice, heavy metal was largely a British creation. The first seeds of heavy metal were sown in the British blues movement of the '60s, specifically among bands who found it hard to adjust to the natural swing of American blues. The rhythms became more squared-off, and the amplified electric instruments became more important, especially with the innovations of artists like the Kinks, the Who, Jimi Hendrix, Cream, and the Jeff Beck Group. Arguably the first true metal band, however, was Led Zeppelin. Initially, Zep played blues tunes heavier and louder than anyone ever had, and soon created an epic, textured brand of heavy rock that drew from many musical sources. Less subtle but perhaps even more influential was Black Sabbath, whose murky, leaden guitar riffs created a doomy fantasy world obsessed with drugs, death, and the occult. Following the blueprint laid down by Zep and Sabbath, several American bands modified heavy metal into more accessible forms during the '70s: the catchy tunes and outrageous stage shows of Alice Cooper and Kiss; the sleazy boogie of Aerosmith; and the flashy guitar leads and wild party rock of Van Halen (not to mention the distinctively minimalist grooves of Australia's AC/DC). In the late '70s, a cache of British bands dubbed the New Wave of British Heavy Metal (including Judas Priest, Iron Maiden, and Motorhead) started playing metal faster, leaner, and with more menace than ever before. They helped influence a new American metal scene known as thrash in the '80s, which took shape as a reaction to metal's new mainstream pop breakthrough, which came courtesy of Def Leppard's Pyromania. Metal enjoyed its greatest presence on the charts during the '80s, thanks to a raft of glammed-up pop-metal bands, but thrash bands played complex riffs at breakneck speed, sometimes dispensing with vocal melody altogether. Thrashers like Metallica and Megadeth built rabid cult followings that pushed them into the mainstream around the same time that grunge wiped pop-metal off the charts. Mainstream metal in the '90s centered around a new hybrid called alternative metal, which (in its most commercially potent form) combined grinding thrash and grunge influences with hip-hop and industrial flourishes, though it broke with metal's past in downplaying the importance of memorable riffs. Meanwhile, the underground grew harsher and bleaker, producing two similar, thrash-derived styles known as death metal and black metal, which produced some of the most abrasive, intense, hyperspeed music and graphic shock tactics the metal world had yet witnessed.