I wanted to expand a little on the trick Sin showed me for making large underground areas, while I have used some in the underdark I wanted to try something different so I opted for a large cave with a pool in it that ended up being made into a mine or way stop of some sort.
Below is a picture from the toolset showing the walkable areas above the ground. The easiest way I have found is to map out the walkable areas at ground zero then use the walkmesh cutter to take out the bits you don't want before lifting them all up above the ceiling. This area was lifted to a height of 35 above the ground but you could go higher or lower if you wished.
Once you have got your basics sorted its pretty easy to get the rest of the area made, in the end I was pleased with the results:
I have uploaded an example of this area to dropbox, while it uses some haks (those from the FRC server) the docks etc are standard placeable so people should be able to see how this area has been made without getting the haks,
