How deep does the Underdark go?

>I'm toying with creating a new fantasy world, and one question that came up
>is--how deep does deep go, in the Underdark?

As deep as you need it to.

>I don't have any standard of comparison--how deep do modern mines go?

I don't  remember, but I think it is in miles.

>How far have modules delved?

The concept is meaningless.   This will be your world and you don't have to
go deeper than anyone else or even stray far from the surface.

>Any opinions?


Unless you are going to worry about your world being a sphere or a plane,
the underlying geophysics can be whatever you want.   Personally I like my
fantasy worlds to be obviously different from reality in as many ways as
possible.   Therefore I would prefer the "Underdark" to be different.

Either there is never a problem with breathing or there is some wierd
eco-system which absorbs methane and carbon dioxide and releases oxygen.
It won't be photosynthesis!

As the GM you need to know the geological history of your world.   How did
it come to be the way it is?   Are there fossils?   Is there any coal?   If
so was it made from squashed trees?
How do gems come into the world?

You can have any answers you like, and you don't have to tell your players
in advance.
But if you don't know, then your dwarfs are going to be a problem becasue
they are supposed to know these things even if the players haven't the
slightest idea.   When it suddenly matters how hard or soft the rock is, or
what would  live on, or what the  are doing
here - you need to talk fast and convincingly.

I can spin details into a land on the fly, but I have to know my frame of
reference first.

Rgds,
Michael.

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