underground map layer(s)

a map that underlays the surface map would be underground layer 1 (UG1). a cave entrance/exit would be needed to get to the surface map and back

once UG1 is a user choice during new game choice adding UG2, UG3, etc is simply a matter of two UG layers that connect via cave entrance/exit

with UG map layers 'movable cave floor' terrain would need to be added to the game

it would interesting if two other new terrains for UG map layers of 'solid rock' AND 'diggable dirt' were also added. with the addition of 'diggable dirt' UG terrain obviously the HK tech tree would need new unit(s) capable of digging the dirt and removing the obstruction to reveal cave floor terrain

having UG layers would then open up the possibility of adjacent districts vertically as well as horizontally. a decision would then need to be made whether UG maps are simply vertically above and below other map layers (like in other 4x games) OR the layers are offset for the new tiles to fit in the intersections of the layers above it and below it similar to real life crystals. I would favor the latter solution but either would make the game more interesting