Alright, this one may seem a bit silly, but it fits. In Season Two of Punky Brewster, originally aired in 1985, there was a bizarre two-part Halloween episode called “The Perils of Punky.” (You can usually find this broken up into several parts on Youtube.)
While you don’t have to be familiar with the tv show to appreciate the episode for its Girls Underground qualities, it does help to know that Punky is an orphan (like many other protagonists) and that she has three regular companions/friends, plus her dog (again, in keeping with the archetype).
In this episode, Punky and her friends go on a camping trip. Her dog Brandon chases a rabbit away and they follow, eventually getting lost and coming to a cave. As they explore the cave (underground!), her friends begin to disappear, one by one, and she must rescue them, and fight the Spirit that is her adversary (whose minions include a giant spider with glowing red eyes that gave me nightmares as a kid). She is also helped by a spirit guide who is the ghost of an Indian princess (please ignore the terrible representation of Native Americans, it was an 80’s children’s program after all), who just happens to also look exactly like Punky. In the end, she defeats the evil spirit with love and goodness – which may seem overly sappy, but actually shows up in a number of Girls Underground stories.
This episode was bizarrely disturbing and dark and stuck with me over the years, which is why I remembered it when thinking of potential Girls Underground examples. When I watched it again, the things that scared me as a child didn’t seem so bad anymore, but instead other elements became much more noticeable, like the scene where she finds her friends dismembered and embedded into the rock of the cave, all still alive. Truly horrifying in a way that only kids programs seem to accomplish.