I play D&D 5e; I am not that familiar with the other editions of D&D. However, I'm looking for lore on the Underdark from any edition, since the settings (e.g. the Forgotten Realms) are still roughly common to most editions (even if certain events have occurred in some editions and not in others).
For context, in my own homebrew universe, I've decided that the Underdark is in fact another plane, although it is accessible from the Material Plane via certain tunnels and such that are like subtle portals (similar to Fey Crossings). However, this question is not about my homebrew universe (which I doubt I'll change regardless of the outcome of this question).
I was looking into the Underdark, searching through information online and in 5e books, to see if the Underdark is a different plane or whether it is simply beneath the "surface" of the Material Plane. It seems as though it's the latter, which means I'll have to go to greater efforts to adapt existing adventures to my homebrew universe that were written in the Forgotten Realms, for example.
However, I believe I got my idea about the Underdark being a different plane from somewhere, so I was wondering if there have ever been any adventures or settings within D&D where the Underdark has been considered a different plane.
I've read online that Matt Colville has used this idea from an adventure called "Night Below", which was apparently an old 2e adventure. I did used to watch some of his videos, so maybe that's where I got this idea from? But even if this lead proves false, are there any adventures or settings that have ever treated the Underdark as a different plane from the Material Plane?