Just played first time underground and its totally out of whack.. specially if you are playing with my mods that increase HP of some units, that will then spawn by digging and just destroy you at the start of the game. 

so list of changes:

Excavation: reduced marauders spawn chance from 15% to 10%. Now they dont attack you, but will just stay idle, like all other wildlife armies. They will still occupy province until you decide to clear them. No more dragons or other very high level units spawned by digging.

Prospecting (industrious culture Scouts) reduced rewards by -50% (now gives around 30 gold or production on average)
if u make 10 scouts and start prospecting whole map you will soon realise how OP it was.

Lava terrain: reduced penalty from -5 to only -1 city stability. Its only terrain type that does that and I dont know why you should be punished like that just for playing underground. So its much lower now.

mod 1.1 version changes for faster moving:
Earthkin transformation (Tome of rock) now gives more bonus walk speed on Rock terrain (5 cost instead of 6)
Cavern floor walk speed has 6 cost instead of 8
River walk speed has 8 cost instead of 12

thats it, if u like it, have fun!

should be compatible with all other mods that dont change exactly the same properties. works on existing saves.

I recommend mod Terraform bedrock. Its nice addition to this one, enables you to break bedrock and connect and expand you underground cities.
NOT included here, download it on st.workshop.

----------------------------- INSTALLATION

as usual unzip files to your "\Documents\Paradox Interactive\Age of Wonders 4\Mods" folder (create one if u dont have)

enable mods in your AOW4 launcher (dowser.exe in game folder)
> playset
> add new playset (needs to be done only once - first time)
> add mod to playset, check if "excavation prospecting" mod is green (enabled) and play.
if you have more mods, just add "excavation prospecting" to your existing playset.