creating circle in the gorund

[{=SuperFly=}]

how do you do that and also how do you put a bunker in the hill

[Sonsai]

Well what I think you're talking about is LoD Terrain.

First make a brush, 512 x 512 is good.

Second go to the top of the toolbar and go to : LOD Terrain > Create from Brush

Third zoom in, and then press "v", these are your vertices

Fourth press Shift and select a row of vertices.

Fifth go upto the top and to: LOD Terrain > Lock/Unlock Vertices

Then just drag any of the other dots to make your crater and such.
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As for the bunker in a hill, I'd just make the bunker, then build some LOD terrain around it. 8)

[nuggets]

that'll only make a crater, if you wanna create a bunker u'll have 2 create it in brushes, then place the terrain around it

[panTera]

you can also make your lod-terrain and then later on place your bunker in it. When your bunker is in the right location then select your terrain and press Shift+F. You'll get a little window: "Edit Facet Flags".
Next, hold down Ctrl+Shift and select the lod-facets (triangles) you want gone. Then in the little window click: "Delete.." and "Done". Now every facet you selected will be deleted in-game (not in Radiant) and you'll have a hole in the terrain for your bunker.