In the final decade of the 1900's, rock music was really starting to turn a corner. After being relegated to vinyl for most of the '80s, the invention of the CD gave artists a lot more wiggle room. Not having to worry about about audio fidelity, artists could do whatever they want, and they really started to stretch in the new decade.
Since you don't have to be confined, some of the biggest artists would end up cramming their records with as much music as they could, oftentimes with varying results. While records like this normally signal little to no quality control over the course of their runtime, every one of these records feels meticulously crafted, as the band tries to really hone in on their skills and present the best version of themselves that they can.
You also have to factor in the idea of the concept album as well, ballooning some of these projects into nearly feature length to try and create the story in your mind as the music plays in the background. The era of conforming to what the needle and vinyl said was a thing of the past now. We were living in the future, and it made for some of the most gargantuan albums that the genre had ever seen.
10. Undertow - Tool
Anyone remotely familiar with Tool's music knows that these guys like to take their time in between releasing albums. After the colossal wait after 10,000 Days, they made sure to make every song count on Fear Inoculum as well, taking the interludes to mess around with different textures and stretching songs out to lure you into a bit of a trance. And when they were at their greenest, they still understood the importance of how to stretch things out.
For most of Undertow, Tool is mining a traditional sounding heavy metal groove, with songs like Sober and Crawl Away delving into something that was a lot more vicious than what we would come to expect on something like Lateralus. While the actual songs themselves aren't that long, the album fades away a little bit longer than expected on Disgustipated, which starts off of as your typical heavy metal song before you're greeted with crickets at the end of the track.
As you wait around for nearly 10 minutes, the album properly ends with an answering machine message from Maynard James Keenan's landlord, complaining about where he was parking his car when he was at home. This was all meant to be lighthearted though, with the track number on the CD counting up and up during the waiting until you finally landed on this song on track 69. Tool would definitely have some more mature themes sprinkled into their songs later, but for right now it was just about taking the piss.