Over the course of his two-decade career, Eminem has become one of the most closely-scrutinized artists in pop music. The immeasurable heights he reached at the turn of the century gave way to an under-explained, drug-induced hiatus in the middle 2000s, which effectively created a microscope under which fans studied his every move. So it's always a surprise when material of any sort is revealed to have slipped through the cracks.

But that's apparently what happened here, with a just-unearthed freestyle from Tim Westwood's show in 1999. The 13-minute clip pairs him with Proof, the late rapper who was Eminem's mentor, friend and frequent collaborator. Everything you'd expect to be on display is right there: the tongue-twisting syllable stacking, the shocking humor, the cartoon violence that's too crude, too vivid.

It was recorded while Eminem was abroad promoting his major label debut album, The Slim Shady LP. That record would vault the Detroit-bred battle rapper into the national spotlight; later in the year, he would be the key supporting player on 2001, Dr. Dre's long-awaited, critically lauded sophomore album. From there, he was off to the pop culture races, becoming, by some measures, the highest-selling rapper of all time and a genuine cultural phenomenon. His most recent album is 2013's The Marshall Mathers LP 2, which hearkened back to his signature work, 2000's The Marshall Mathers LP. Late last year, a similar unreleased freestyle from 1998 was discovered, that one featuring Royce da 5'9".

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