Traditional Cable television is becoming a dying relic as Streaming networks like Netflix and services like YouTube overtake it in mass market popularity. That said, Its fun to look back at the once novel idea of cable television. The idea of more channels, many of which covered a specific theme or targeted a particular audience was what drew people to it in the 80s, 90s, and even the early 2000s. Because of this, there were some cable channels that felt distinctly "underground". In terms of aesthetics, programming, marketing, even the commercial breaks, where they defied mainstream television wisdom, and catered to really particular audiences with very particular focuses.
Which cable network growing up, did you think felt the most "underground". As in, not mass produced, not super popular. Having a weird, niche feel, with weird, niche programming, and generally having an almost, Avant-garde style to it? The champion of this for me was and always will be Adult Swim. Say what you will about Adult Swim now, It's amazing how relevant they've managed to keep themselves after all these years, even as the Cable landscape shatters, just by playing by their own rules. Love them or hate them, they're not afraid to be different.
For me, as a Finn, it was definitely Moontv. It ran from 97-03, and it fully embraced the term underground. the channel showed pc and console game reviews, cooking shows, music festivals, movie reviews and later started showing adult films as well.
The problem with the channel was how unpredictable it was most of the time. For weeks they would only show reruns and suddenly you'd see new shows and even new episodes!
Oh, and from what I've heard, alcohol was not frowned upon while interviewing guests.
I can only think of one, but not really 'underground' in the way the OP describes.
MTV in the late 80s, at least around these parts.
The town I grew up in was pretty damn conservative and religious, as was often the case in small rural Flemish towns, and certainly in the 80s and early 90s. Our neighbours happened to be pretty much the most conservative for kilometers around. The family living next door to us had three sons. The youngest was Danny. He loved playing guitar, listening to hardrock & metal and partying it up. Imagine your typical 80's metalhead. That was Danny. As you can probably guess, he was the black sheep of the family. Danny wanted to watch MTV, but his parents wouldn't let him at home, since MTV was a moral danger that poisons the minds of young people with the sex, drugs and rock 'n roll.
Luckily for Danny, my family was breathtakingly progressive compared to our neighbours and my dad was also a musician. Played in a Lynyrd Skynyrd cover band for years. He knew well enough that "rock 'n roll is the devil's music" stuff was BS. So for a few years, my parents would let Danny come over to our place to watch Headbanger's Ball, until he finally moved out. I was only about 3-4 years old back then, and I didn't understand what was going on. I just thought Danny came over to play Legos with me before my bedtime. Wasn't till years later that I learned the real reason.
Apparently Danny's story was not uncommon. MTV was pretty controversial at the time. That shit wasn't a US exclusive.
I don't remember the TV network, but back in the 90s, I saw commercials for a TV show called Las Vegas Showgirls. It wasn't censored. That was very impactful on my teenage mind. Certainly not the kind of thing that you'd regularly see. It was basic cable, too.
...I also remember years later they started blurring it. Sad days.
...yes, I know that you'd see far more, uh, "impressive" things on the pay channels. I very much treasured a videotaped copy of Rapa-Nui that I obtained during a free weekend of Showtime.
//That was the best that I could come up with on the spot