This helps more mission makers, than it does just players ...
But I found how to do a nifty "trick" to add in "underground areas" , you normally wouldn't have avaliable.
It's entirely "faked" , but effective ...
Lets say you got a building, like the "bunker doors" which doesn't do anything ... or any "structure" that looks like it should go underground ... like a basement door, that you can't open ... ect ...
What you need is a Trigger placed on the door AREA , where it will "addaction" to players menu once inside the trigger radius.
Have it say " go inside " or "down" or something like that.
Choosing the option, you need a script to simply teleport the player to a marker or Way Point , call that marker or way point something Underground1. You need a second marker , where you placed the entrance trigger, named Back_out1.
Then you need interior buildings ... I would use something like MGB Killhouses , as most of those are concrete block looking.
Out in debug area ( somewhere flat ) or some unused part pf the map ... place and line up the buildings to be "continuous" , connected by hallways ... you want to give the illusion when inside, you are "underground."
This might require blocking off some windows or doors you don't want the "outside" shining through.
To go back, you need another trigger near your "exit" , which addaction to player menu , call it like "return" or something ... have it fire your teleport script to marker or way point called "Back_out1". You place the marker/way point called Underground1 here in this location.
So for instance, just as an example ... you put a "bunker" on top of Klen Mountain.
When you get near the door, the scroll menu gains the option to "go in", clicking it fires a telport that takes you inside the custom crafted and blocked off area.
Giving the "illusion" , you simply went under the ground , which actually teleported you a long ways away across the map, to an interior of an above ground area, you placed and made the area to your liking.
Going to it's "exit" , triggers another menu action, say " go back" ... which simply teleports you back across the map , at the outside of the entrance to the "bunker".
It's actually way easier to do all that, than I explained or made it sound. The hard parts I think, would be making the "underground" part look right, and block off any exterior means to look outside.
But is possible to do.
I had waited for map makers in Arma to do actual underground maps ... I too wished for a Metro like underground tunnels and systems.
I made source SDK maps in hammer a few years ago, maps for half-life player fan based custom content, maps for gmod, a few others just messing around/learning with hammer.
In that, underground is easy to do ...
Map making , as in actually creating a map for Arma2 ... is a huge undertaking , and making underground areas is near impossible. In order to do so, you have to "trick" the map program, into thinking you are working "above ground level", and after completion, "sink" the area precisely where you want it. It doesn't give you any tools do this, you just got to micro-adjust position, by the coordinates numbers. but you can only go so far "down" that way.
I looked into it before , as I do not really like most Arma2 maps/islands ... or they tend not to fit my ideas needed at the time.
I also seen somewhat how to combine buildings and parts from the regular Arma buildings , I think you have to script em to be "no clipping" or "no collide" ... but you attach and turn the buildings , add in some entrance houses, ... surround the entire creation by some sort of walls, like shipping containers ...
Can make a HUGE indoor complex, that resembles some sort of large factory , or warehouse. Just takes a good eye for design , and knowing all the ladders, and doors need to go somewhere.