Few years changed heavy music as dramatically as 1991. Established metal bands reached enormous new audiences while the underground became more extreme, technical and adventurous. Thrash, death metal, progressive metal, hard rock and the heavy side of Seattle’s alternative scene all produced landmark releases. Instead of moving in one direction, metal expanded at both ends: more accessible at the top and more uncompromising below.

Metallica’s self-titled fifth album—quickly known as the Black Album—was the year’s defining commercial event. Working with producer Bob Rock, the band shortened its arrangements, slowed many tempos and placed greater emphasis on weight, melody and direct songwriting. “Enter Sandman,” “Sad but True,” “The Unforgiven,” “Wherever I May Roam” and “Nothing Else Matters” carried Metallica far beyond thrash audiences. Some longtime fans rejected the change, but the album turned the band into a global stadium act and brought millions of new listeners into heavy metal.

At the opposite edge of the genre, Death transformed technical metal with Human. Chuck Schuldiner recruited guitarist Paul Masvidal, drummer Sean Reinert and bassist Steve Di Giorgio, surrounding himself with musicians capable of taking Death into progressive territory. “Flattening of Emotions,” “Suicide Machine,” “Together as One” and “Lack of Comprehension” combined aggression with remarkable precision. Sepultura’s Arise made a similarly important advance, joining thrash intensity, death-metal weight and increasingly atmospheric production on the title track, “Dead Embryonic Cells” and “Desperate Cry.”

Death metal experienced one of the strongest years in its history. Carcass added guitarist Michael Amott and moved beyond grindcore on Necroticism: Descanting the Insalubrious, building longer and more intricate songs without losing its gruesome identity. Morbid Angel’s Blessed Are the Sick mixed speed, doom and classical influence into a darker follow-up to Altars of Madness. Entombed’s Clandestine refined the Swedish buzzsaw sound, while Cannibal Corpse’s Butchered at Birth pushed graphic extremity far beyond mainstream tolerance.

Heavy music was also changing outside traditional metal circles. Soundgarden’s Badmotorfinger joined unusual time signatures, Chris Cornell’s extraordinary voice and Kim Thayil’s dark riffs on “Rusty Cage,” “Outshined” and “Jesus Christ Pose.” The album helped prove that Seattle’s emerging alternative movement could be every bit as heavy as metal. Skid Row also rejected an easy commercial repeat with Slave to the Grind, delivering a harder record led by the title track, “Monkey Business” and “Quicksand Jesus.”

Savatage created its most ambitious narrative with Streets: A Rock Opera. Producer and co-writer Paul O’Neill helped Jon Oliva, Criss Oliva, Johnny Lee Middleton and Steve Wacholz shape the rise, collapse and redemption of fictional rock star D.T. Jesus. Songs including “Jesus Saves,” “Strange Reality,” “Believe” and the title track joined theatrical storytelling to emotional heavy metal.

The year was far deeper than any Top 10 could show. Ozzy Osbourne’s No More Tears produced one of his greatest title tracks, Guns N’ Roses issued both volumes of Use Your Illusion, and Overkill, Van Halen, Armored Saint, Dismember, Suffocation, Bolt Thrower and Fates Warning all delivered important records. By the end of 1991, the old division between mainstream and underground metal had become a vast landscape of competing sounds.