Cut out everything within a volume. Is it possible? I have a very large model of terrain, spanning several thousand kilometers, with meshes of buildings and trees scattered across it. I'm only interested in one single area of the terrain, and I want to cut out a cylindrical volume around the area, and delete everything outside the volume. I'm familiar with the knife tool, but that's on a per-mesh basis, same with the boolean modifier.

Is there any way I can achieve this?
I have no such a big terrain here ... but this should work (with blender 2.8 at least...I don't know whether older versions will work).

Create the cylinder and position it at the correct place.

Select a mesh on the terrain inside the cylinder.
Add a boolean modifier and set it to intersect. Check the result.
If ok select everything and deselect the cylinder then.
Select the mesh from before.
Copy the boolean modifier with "copy selected modifier" to all the meshes.
deselect everything. Select the mesh from the start. 
Press ALT and then APPLY of the boolean modifier.
Make a backup of your blender file before :)