Has the Underdark ever been a separate plane to the Material Plane?

Question

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?

Answer 1

It appears in Matt Colville's homebrew campaign, but not D&D canon

In his Campaign Diary 010, Matt Colville describes that in his setting the Underdark is not just physically underground, but somehow a dimensionally separate world:

He provides the Chain with what is basically an interdimensional map that they could use to get from this world to the World Below, which is where the adventure happens. In my setting, the "Underdark" is called the World Below, and it's another world; it's another realm.

Note that in a previous version of this answer I said that Matt had previously described his Underdark as if it was merely physically below the Earth. However, it's plausible that Matt has described his Underdark as another world before this question was asked, and it merely hadn't appeared in any of the videos or content I was aware of.

However, not in canon

The list of canonical Dungeons & Dragons books is finite, and of those, the only works likely to define the Underdark in any canonical sense are campaign setting books, setting-neutral adventures taking place in the Underdark, guidebooks to the Underdark, and magazine articles. Novels in a given setting are unlikely to contradict their sources.

Many settings either do not have a planar cosmology at all, or do not even have an Underdark, which was largely a Greyhawk-specific feature adopted by the Forgotten Realms in AD&D 2e and later settings from 3e onward.

This lets us narrow down the possible sources to something we can manage.

Setting-specific

We can generally assume that setting lore remains constant between editions.

• Greyhawk: The most recent source is the Living Greyhawk Gazetteer (2000), p. 18, which mentions "the subterranean Underdark", containing the Vault of the Drow physically located beneath the Hellfurnaces mountain range.
• Forgotten Realms: The 3e book Underdark describes it as a series of caverns "that riddle the earth beneath Faerûn". The 4e Forgotten Realms Campaign Guide describes it nearly identically as "a network of subterranean tunnels, caverns, seas, and rivers that spans the entire world". The 5e Sword Coast Adventurer's Guide, p.99, affirms that in the Forgotten Realms, the Underdark is still physically beneath the surface of the world. The Al-Qadim and Kara-Tur settings are parts of the Forgotten Realms.
• Eberron: According to the Eberron Campaign Setting, the Underdark is largely synonymous with Khyber, again, a place physically below the earth.
• Dark Sun: The 4e Dark Sun Campaign Setting mentions the Underdark only in passing, but describes on p. 116 that planar travel is rare, and the Underdark does not appear in its cosmology described on page 17. Its AD&D 2nd edition counterpart, the Dark Sun Campaign Setting, does not mention any Underdark.
• Ghostwalk: The Underdark is mentioned only once in passing.
• Ravenloft: Sequestered from the rest of the planes, Ravenloft effectively has no planar cosmology into which a Plane of Underdark could be placed.
• Hollow World: Spells for planar travel do not work, meaning that Hollow World effectively has no planar cosmology.
• Spelljammer, Planescape: These works expressly did not contradict Greyhawk or Realms planar cosmology.
• Nentir Vale: In the "World Axis" cosmology that was part of the nameless (but fan-nicknamed) core setting of 4th edition, the Underdark is portrayed as functioning very similarly to a transitive plane. Simply exploring its tunnels can bring you from or to any of the following planes: the mortal world (Material Plane), Feywild, Shadowfell, Elemental Chaos, Far Realm. It is not explicitly described as a transitive plane, but such terminology is not really used in the World Axis anyway.
• Other settings: Birthright, Dragonlance, Jakandar, Mystara and Rokugan either have no Underdark or have no established planar cosmology in which an Underdark could be placed.

Setting-neutral

• The AD&D Dungeoneer's Survival Guide asserts that the Underdark consists of subterranean caverns deep beneath the earth.
• The 3.5 Dungeon Master's Guide, p. 59, defines it as "a subterranean world that is one enormous natural cavern complex running under the surface of entire continents".
• The 4e Dungeon Master's Guide, p.106, defines the Underdark as a "sprawling networks of natural caverns stretching deep below the earth". This is canonical for the Points of Light setting and generic sources.
• The 5e Dungeon Master's Guide, p.101, describes that mining too deeply can break into the Underdark, which suggests that it's physically located on the material plane.
• Night Below insists that it has been written to integrate into any campaign setting. It suggests that the Underdark is located beneath the world.

Other sources

• The DragonDex lists all articles appearing in Dragon magazine which reference the Underdark. Of those, only one describes possible alternate Underdarks, and none of them are extraplanar in nature.
• As best I can search Wikipedia and Google, there is no reference to the Plane of Underdark.

Answer 2

While the Underdark has consistently been represented as part of the “world” or “prime material plane” through editions, there are some ways it has been portrayed as like another plane.

Generally, a different plane in D&D is a place where you need a portal or a spell to get to, while your character can indeed, simply walk into the Underdark.

The rules of existence in the Underdark are sometimes different than the world

Sometimes the Underdark is represented as a place where the rules of the surface world do not apply — in a way that is similar to another plane.

These differences were pretty stark in fourth edition. For example, the Underdark is contrasted to “the world” in a way that makes it sound quite a separate place:

But unlike the world, the Underdark remains largely untouched by the gods’ refining efforts. It is a rawer creation that owes its impermanent substance and warped supernatural nature to the haphazard efforts of the early primordials.

PH, 4e, p. 6 (“Origins of the Underdark”)

Passages in the Underdark would expand or close up (except for The King’s Highway) where “normal” cave passages would not:

Many types of stone found in the Underdark are constantly growing and reshaping themselves. In the space of a century, a year, or even a week, an area of living stone can take on a different form…

PH, 4e, p. 13

Parts of the Underdark have been separate planes

In fourth edition, the Feydark and the Shadowdark were part of their perspective planes (the Feywild and the Shadowfell) and thus on a separate plane from “the world.”

A series of tunnels known as “The King’s Highway” connected the Underdark to the other underworlds of the shadow planes (PH, 4e, p. 12) so it was possible to walk right onto a separate plane of existence, without the aid of magic, or a portal.