Metallica were arguably the biggest underground band in the world when they set upon making the follow-up to 1988's ...And Justice for All, but the creative and commercial leap they made with their 1991 self-titled LP — better known as the "Black Album" is still mind-blowing, even in retrospect. Justice's sprawling, intricate, multi-part political epics were focused into concise, personal heavy pop songs; frontman James Hetfield actually started legitimately singing, not just yelling; and the production just popped, especially compared to its predecessor's virtually bass-less mix, making songs like "Sad but True" and smash lead single "Enter Sandman" simply explode through speakers and headphones.

On the commercial side, the Black Album was a massive hit at the time, catapulting Metallica to full-fledged superstardom, but what's even more remarkable is its staying power. The record continues to outsell major new releases every week and stands as one of the best-selling albums of all time of any genre. "It was like your older brother went to college and became Bill Gates," Anthrax's Charlie Benante said to Revolver in 2011, looking back on his friends and fellow thrashers' unimaginable breakthrough. "It was like, 'Make sure you write sometimes please.' And that was it. No longer was there really the Big 4. Metallica was this thing unto itself. They were this huge entity. When they have a record like the 'Black Album,' when something is like Back in Black or Dark Side of the Moon, that's it. That's it, you know, goodbye. You don't need anything else."

Here, in celebration of the record that turned Metallica into a "huge entity" are 10 things you likely didn't know about the Black Album.

1. The Black Album's shorter, more streamlined songs were a reaction against ...And Justice for All's proggy excess

"​​​​​​To me, the …And Justice For All album sounds horrible, awful, can't fucking stand it," James Hetfield told Uncut bluntly in 2007. "That was our fancy stage, showing off too much. We knew we had to move on and the Black Album was the opposite. So when me and Lars got back together after a short break, I said, 'We gotta really try and write some shorter, to-the-point songs.'"

Hetfield wasn't alone in feeling like Metallica had gone too fall off the proggy deep end with Justice. "We realized that the general consensus was that songs were too fucking long," Kirk Hammett revealed to Rolling Stone in 1991. "Everyone [in the crowd] would have these long faces, and I'd think, 'Goddamn, they're not enjoying it as much as we are ... I remember getting offstage one night after playing 'Justice' and one of us saying, 'Fuck, that's the last time we ever play that fucking song!'"

Video of Metallica - Enter Sandman [Official Music Video]

2. "Enter Sandman" was originally about crib death

The Black Album's opening cut and lead single, "Enter Sandman" is worthy of it's own "Things You Didn't Know" piece (and guess what? here it is), but one of the most surprising facts about the song is its extremely off-putting original theme: crib death. "Y'know, baby suddenly dies, the sandman killed it," the Hetfield described it in 2007. The lyrics were so grim — including the line "Disrupt the perfect family" instead of "Off to never never land" — that Bob Rock and Lars Ulrich stepped in. "I can remember when I wrote the lyrics to 'Enter Sandman,' Bob Rock and Lars came to me and said, 'These aren't as good as they could be.'" the singer recalled to Guitar World in 2008. "And that pissed me off so much. I was like, 'Fuck you! I'm the writer here!' That was the first challenge from someone else, and it made me work harder."

3. The guitar solo on "Enter Sandman" was inspired, in part, by a sample on an Ice-T album

"I think the time has come to reveal where I actually got the guitar lick before the breakdown in 'Enter Sandman,' Hammett told Guitar World. "It's from 'Magic Man,' by Heart, but I didn't get it from Heart's version. I got it from a cut off Ice-T's Power album, where he sampled it. I heard that and thought, I have to snake this!"

4. "Sad but True" was originally written in a higher key and, as a result, wasn't nearly as crushingly heavy

When Rock heard the demo for "Sad but True," he thought it was "the Kashmir of the Nineties," referencing the classic Led Zeppelin song. "The riff was astounding," he said to Guitar World. "Rhythmically, I could tell it had the potential to be absolutely crushing!" Then, while in pre-production, he noticed that all the songs that band has brought in, including "Sad but True," were in the key of E. "I brought this to the band's attention, and they said, 'Well, isn't E the lowest note?'" Rock recalled. "So I told them that on Mötley Crüe's Dr. Feelgood, which I produced and Metallica loved, the band had tuned down to D. Metallica then tuned down to D, and that's when the riff really became huge. It was this force that you just couldn't stop, no matter what."

5. "Nothing Else Matters" was not intended to be a Metallica song or to be played for "other people"

"It was a song written in hotel rooms on [Justice] tour about missing friends at home, being out for such a long time," Hetfield told Jeff Woods in 2016. "That was a song that was not meant to be played for other people, it was for me. I think that's important — to write music that makes you feel good, I've got quite a few songs that are like that. 'Nothing Else Matters' was heard by the band, they thought it was amazing. I though, 'You're crazy. That's just this sappy ballady thing that makes me feel OK.' 'No, that's good!'"

That said, at least one other Metallica member was initially uncomfortable with the song, as well. "All I could think of at the time was, James wrote a fucking love song to his girlfriend?" Hammett said to Playboy in 2001. "That's just weird."

Video of Metallica - Nothing Else Matters [Official Music Video]

For his part, Hetfield was so unsure about releasing "Nothing Else Matters" as a Metallica song that when the band debuted the Black Album with a free listening party at Madison Square Garden on August 3rd, 1991, he was afraid diehard fans might become physically ill upon hearing it. "I was just waiting for 'Nothing Else Matters' to come on," the singer recalls in the book Metallica Unbound. "You know, to see if these people just look at each other and throw up!" To his surprise and relief, "people were pretty into it, which," he adds, "was pretty amazing."

6. James Hetfield's vocals on "The Unforgiven" and "Nothing Else Matters" were inspired by Chris Isaak

Hetfield's singing took a huge step forward on the Black Album, evolving from a primal thrash-metal yell to a much more nuanced and wide-ranging rock vocal. When making that leap, the Metallica frontman looked to Chris Isaak, particularly his song "Wicked Game," which was a massive early-Nineties hit, and with which Hetfield "was very enamored," according to Rock. "[Hetfield] played me a Chris Isaak record, and he said, 'On 'Nothing Else Matters' and 'The Unforgiven,' I want to sing. How do you sing like this?'" the producer recalled on Chris Jericho's Talk Is Jericho podcast in 2015. "I said, 'I'll get you a great vocal sound, so you don't have to double your vocals. What you hear in Chris Isaak's voice is the nuances when he sings — he isn't doubled. He's actually performing. You perform.' We set it up so he was comfortable and had a great vocal sound, and then he sang. Every day he got better, and he got comfortable with it. He became a great singer."

7. Bob Rock thought the lyrics to "Of Wolf and Man" were "silly"

"I'll be honest: at first I thought it was silly to write about a wolf," Rock said to Guitar World in 2011. "I was like, 'Oh, great, a song about a wolf. What are you fucking getting at? May as well write about pyramids or something.' When metal goes in these kinds of areas, I lose the plot." He eventually came around. "Then, as we got more into James's lyrics, I realized that the song wasn't silly, that there was an earthiness to it," he recalled. "We talked about making the song go through a transformation, kind of reflecting the lyrics. It took a while. I'm not sure if we got there fully, but we got there most of the way."

8. Lars Ulrich, Kirk Hammett and Jason Newsted were all going through divorces during the making of the Black Album

The personal lives of three of the four Metallica members were in deep disarray as they set upon writing and recording what would prove to be the group's breakthrough album, and that darkness and chaos may have crept into the resulting music. "Lars, Jason and I were going through divorces," Hammett revealed to Playboy. "I was an emotional wreck. I was trying to take those feeling of guilt and failure and channel them into the music, to get something positive out of it. Jason and Lars were, too, and I think that has a lot to do with why the Black Album sounds the way it does."

Video of A Year And A Half In The Life of Metallica (full disc one)

9. Ulrich came up with the idea of going very stark and simple with the cover art as a reaction against standard, cartoonish metal imagery

According to the book Enter Night: A Biography of Metallica, Ulrich was struck with the initial guiding vision for the Black Album's cover art early in the recording process. Author Mick Wall writes that the drummer was "browsing through a typically colorful heavy metal mag, noticing how the ads for various albums all looked the same," when he realized Metallica had to do something different, Ultimate Classic Rock pointed out. "All these cartoon characters and all this steel and blood and guts," Ulrich said to Wall. "It was like, 'Let's get as far away as possible from this.'" The resulting all-black cover certainly made a statement, which Hetfield expressed succinctly as, "Here it is, black sleeve, black logo, fuck you."

10. Metallica and Bob Rock found working with each other so difficult that they planned to never do it again

If you've ever watched the incredible documentary A Year and a Half in the Life of Metallica, you know that the making of the Black Album was a tense, contentious affair. "I used to call James Dr. No," Rock told Rolling Stone. "Whenever I was about to make a suggestion that seemed even a little off the wall, he'd say no before I'd even finished the first sentence." Indeed, things were so tense during the recording sessions, that the producer "told the guys when we were done that I'd never work with them again. They felt the same way about me." Of course, things worked out a little differently. After the album's smash success, Rock and Metallica would go on to collaborate over three more albums, Load, Reload and St. Anger.