I thought I would expand on Rhye's Underworld Terrain, which was a simple palette alteration, to make it more 'underworld' like.
First off, some "forests" that fit the theme, using sprites from Ultima VIII:
Stalagmites replace jungle, giant mushrooms replace forest, and pine forest is replaced by a mix of living and dead 'normal' trees.
underworld forests
Second, a recolor replacing the lava in Rhye's graphics mod to look like water, for your underground lakes and springs.
water recolor
Some related files:
mushroom crops
skeleton pollution
Now I just need to make a Ultima 8 mod... eventually.
Enjoy
edit: fixed link for water: http://forums.civfanatics.com/downloads.php?do=file&id=7652
First off, some "forests" that fit the theme, using sprites from Ultima VIII:
Spoiler :

Stalagmites replace jungle, giant mushrooms replace forest, and pine forest is replaced by a mix of living and dead 'normal' trees.
underworld forests
Second, a recolor replacing the lava in Rhye's graphics mod to look like water, for your underground lakes and springs.
water recolor
Some related files:
mushroom crops
skeleton pollution
Now I just need to make a Ultima 8 mod... eventually.
Enjoy
edit: fixed link for water: http://forums.civfanatics.com/downloads.php?do=file&id=7652