Thanks everyone for the input thus far. I'll definitely take those geographical facts into account before I start the final rendition. I did most of my major rivers based on very distant-range topo maps - so catching all the smaller details is pretty tough. So I appreciate all the extra eyes and first-hand knowledge shared.

Due to writing/storyboarding necessities I had to take a break from working on the surface map and define some of the underworld locations and regions. So I basically just took the surface map and overlayed some geo-technical maps that show where all the real-world Karst regions are for the Mediterranean. (Karst is a term for land that is underlain by limestone and has lots of sinkholes, fissures, and underground waterworks. That would also make it the most probably type of land for natural cave formation.)

So here is a quick glimpse at a VERY early (and I emphasize the VERY) definition for the regions of the underworld - most of the larger regions will be interconnected, both naturally and by supernatural craftsmanship as many of the fallen angels have established the underworld (or as it's more specifically known as: The Shadowlands) as their own earthly kingdom.

Click image for larger version.  Name: CotFS Underworld Map v1.jpg  Views: 1527  Size: 3.63 MB  ID: 58510

Again - it's very preliminary, and the faded underlay of the surface world map is purely for reference. More than likely I will only be doing smaller regional maps of the Shadowlands - rather than one big massive world map like I am for the Brightlands (the world above). Most of the caves and caverns will be naturally formed from underground rivers that follow (generally, but not exactly) the rivers and water networks above them.