I'd love to see an "underground" realm, You can't easily get there, I see it as a late game thing.
Like suggested, the entrance isn't visible and should be located using some sub-terrain scanning devices.
Once a "Entrance" is found one can enter and leave the underground at wish, but my idea is that we're going miles down.
If you dont want to take a long time to get there one would like to build an elevator, and power it ofc.
Down there it's Hot, toxic, and a whole different range of creatures live there.
One cant survive there without a solid defence against the eviorment as well as the enemy. Buildings need to bee cooled if they dont want to melt under the workload of excavating gems.
Turrets will need to be cooled, if not they damage them selves by overheating.
Acidic lakes will block your path, cave-ins will make everything even more risky, you will need to use your flying robots to keep alive, ready for the fight.
Your activity underground however will lure THE MEGA WORM, after 30 minutes or so he will attempt to eat your (irritating vibration of your miners, shooting turrets etc) stuff.
This mega worm does LOADS of damage realy fast, like swimming through your undefended base, leaving destruction behind. He's well defended against the heat of laser turrets (yeah cuz he lives in the hot part) but is easily damaged by ice cold bullets...
Pollution of machines will trap heat, overheating your buildings even faster, and lure more enemy's. So you'd like to ventilate your pollution out to the "overworld".
One will need a special suit to survive some time down there. It'll need to cool you down, give you air, and light!. And as I said be4, things are toxic down there, acidic even. Exploring the area will damage the suit, drain your shields. So you'll need to recharge your suit regularly or it'll melt off.
You'll find special resources there that will help you tech up further.
- gems that are used for light-based-computers (instead of "slow" transistors), better lasers, laser Drilling mining. Plasma guns?
- Heavy ores made to strengthen/build tanks, Heavy pressure Buildings, gauss cannons?
xtra idea: These underground caves are not infinite however. Once found it's a limited area of interest.
Go in, get to the stuff, start mining. But however lose your base before mining things out entirely. One can face Hell but not expect to conquer it.