[Request/Idea] Underground

[scumbag]

I'm going to assume a lot of people here have played a certain "Dorf" game.
What I think RimWorld needs is the ability to dig under ground, or upwards into a mountain. It would add so much more depth (lol, literally) to base builds.

It could add a few research items as well:

The deeper you go, the harder the soil/stone will be. You have to research better pickaxes to be able to go deeper.

Depth|---------------------------|Mining Level:Time/Cost
     00|---------------------------| Lv0:0 (surface level)
     01|---------------------------| Lv1:500
     05|---------------------------| Lv2:750
     10|---------------------------| Lv3:1000
     15|---------------------------| Lv4:1500
     20|---------------------------| Lv5:2000
     25|---------------------------| Lv5:2500
     30|---------------------------| Lv6:3000
     35|---------------------------| Lv1:3000
     40|---------------------------| Lv2:3500
     45|---------------------------| Lv3:4000
     50|---------------------------| Lv4:5000

Different ores could spawn on different levels.

Key:
* - very rare
** - rare
*** - common
**** - abundant
***** - everywhere
Depth|Ores
00-05: Steel**
06-10: Steel***
11-15: Steel***, Silver**, Gold**, Uranium*
16-20: Steel*****, Silver**
21-25: Steel*****, Silver*** (common)
26-30: Steel*****, Silver****
31-35: Steel*****, Silver****, Gold*
36-40: Steel****, Silver****, Gold**, Uranium**
41-45: Steel****, Silver****, Gold***, Uranium***
46-50: Steel****, Silver****, Gold****, Uranium****

Another thing that could be added, deadly cave-ins. To avoid this you have to build support beams/walls in large caves. If the ceiling collapses, the floor above (if there is one) should also be destroyed.
New floor tiles and constructions could be added, like: stairs, ramps, elevators, holes, pungee pit, drawbridges, etc.

I think that covers pretty much all of it. Thanks for reading. If have anything to add, let me know!

Thanks,
-scumbag