removing backgrounds

[Tomt478]

Is there a way to remove a "flatten" background from just a certain area? In other words, I have a "flatten " poly under my airport that covers the entire airport, I need to remove that flatten around only certain areas so as to build a mountain in those areas. I cannot build the mountains with a flatten under them so I need to remove the flatten without having to completely remove the entire flatten, build the mountains then have to replace the flatten under only those parts without the mountains (that would be a lot of hard work). I need a simpler solution to build mountains on top of what I have without having to painstakingly redraw the flatten poly that covers the entire airport. this is not a stock airport it is one of my own design, so there is no "google map" location in the real world for it. The airport is located in the mountainous region of new mexico which is the reason for the flatten poly. I had to flatten the land then start placing my airport. I have been working on this project for a long time adding land classes and such. I just want to make it more interesting using some mountains. I use my simulator as a visual map for my current D&D scenario and so far I have things working pretty good, I just want to add some new terrain elements to things to make it more interesting for the players. It took a lot of work to modify the game itself to include modern world elements such as planes and cars which normally don't exist in a D&D world, but my players are having a ball with it so I want to make things a bit more challenging by adding some unexpected elements and mountains are one of those things I would love to add.