Since we wont see, well most likely, underground areas in Elemental, how about this;

At the start of the game you can choose you rase to be a subterran rase, mutch like in MOO.

This would only be a cosmetic change, but perhaps with some bonus and some disadvantage, like more protected

but less people, more chance of iron, but less chance of fertile land

At the start your city is say 30% subterran, with time you can choose to extend deeper into the earth or grow wider

out on the surface, max would be about 80% subterran, (minimum would be 20%),

with huge bonus to defence, but little ways to expand into a big city, it might be a lvl 5 city,

but with only 20% the people in it

You would not be able to walk or travel underground in your city, just that you city would be different from other more

normal ones.