How deep is the Underdark?

[Gilladian]

In my campaign world, the "upper levels" of the underdark are maybe a mile thick. That's "anything attached to the surface" and includes most dwarvish kingdoms.

Below that the middle realm is a couple more miles thick. There's a very little hint of the core radiance there, which helps some things survive and mutates many others.

Below that are the deep underdark where many many things reside that are fearful of both the hollow core and the surface. Most underdark kingdoms are strongest here, but may also connect to the middle and the core.

At the core is the body of a god, which emits a bitter radiation that causes much evil to exist, including things like ixitchachitl and beholderkin, etc... The "core" is a burning starlike object that "lights" the hollow earth, which is many miles in diameter.

 

[Empirate]

How is it possible that nobody has mentioned either 'Drow of the Underdark', or, even more logically, the supplement simply and aptly named 'Underdark'?!

The latter states that the Lowerdark (lowest layer of the Underdark) can in places go as deep as 40 miles, although it is less a layer than a catchall category for all the really deep places, because these are seldom interconnected. There's a lot of talk of interplanar connections in the Underdark, as well, most of them to the Elemental Plane of Earth, to Pandemonium, or similar earth/cave-themed planes.

 

[Yora]

In the real world, the crust of the earth has a thickness that ranges from about 25 to up to 70 km, or roughly 15 to 45 miles. On average, temperature rises by 3°C for every 100 meters of depth. The deepest mine on earth goes almost 4 km down and it's relatively cool, with the rocks being 60°C hot.
I have no clue if being below high mountains makes a difference. Mountains are formed by crust being pushed upward and hot rock probably wouldn't cool down just by being 4006 miles away from the core rather than 4000 miles.

Miners keep their body temperature below 40°C by sweating, which means they need a lot of water. And that water probably would have to be cooler than 40°C or you add more heat to the body than you lose.
This could be compensated by ground water comming from the surface. But why the underdark never fills up with water is almost never explained in most settings.

 

[RUMBLETiGER]

How is it possible that nobody has mentioned either 'Drow of the Underdark', or, even more logically, the supplement simply and aptly named 'Underdark'?!

The latter states that the Lowerdark (lowest layer of the Underdark) can in places go as deep as 40 miles, although it is less a layer than a catchall category for all the really deep places, because these are seldom interconnected. There's a lot of talk of interplanar connections in the Underdark, as well, most of them to the Elemental Plane of Earth, to Pandemonium, or similar earth/cave-themed planes.

I have and searched each of these books and haven't yet found the answer to my question. In the Underdark book I checked Chapter 7 and the beginning of chapter 8, so please let me know where you found that info. In Dorw of the Underdark I poked around a little in chapter 6 and didn't see anything.

EDIT: I FOUND IT! Underdark p. 117, I just needed to read more into chapter 7. Thank you [MENTION=78958]Empirate[/MENTION]!

 

[Gilladian]

Alas, I have never run a campaign wherein the underdark/deepearth came into play. I need to!

You can read about it in a little more detail here:Vishteer Campaign / Miraborian Geography

 

[Ashtagon]

I'm aware of the difference between the Prime Material and Baator.

The way things are going, I'm not so sure there's much of a difference any more.