Sub Terror(Chapter.2 Q.3) Art Bible 1

This is the first Concept background i drew just to get the look for now still a work in progress and still working on character concepts.

First underground city 

 This is basically the first look at what i want to capture in my game idea this is a view from outside the main gates of the city and wanted to capture a certain Bioshock reference, in which you the player get your first look of this what seems impossible city

These are references that i used the URL's are below:
http://www.askipedia.com/how-many-underground-cities-are-there-and-where-are-they-located/underground_city/

http://weburbanist.com/2007/10/15/7-more-underground-wonders-of-the-world-lost-caverns-and-cities/