@deandddd Happy to help...yes definitely most of the action is inside for sure.
The Craters seem part fortifications and part natural. They ALL have a purpose and are used to form energy of sort (probably plasma) or funnel materials through the network. If you look at the bottom left tunnel it covers vast areas. Nothing really travels much on the surface and when it does they go straight into trenches and tunnels.
Heres a link to the larger file...
I will try to check my archives an hope to fill in some of the other crater posts on the journey...
M