Looks like I am the only one having this problem.
I need to eliminate the gap between the terrain and sky (attached). All tutorials are using mountains. I can’t use because it is a representation of real plain. I tried placing billboards on a distance but they couldn’t hide the gap.
I am using Physically Based Sky on Unity 2019.4 HDRP.
Any ideas?
You could use a basic fog volume.
-Set your base height to terrain Min
-Max height to terrain/building (or creative ) max -set your dog max distance cull to that of your camera’s and it should blend.
If you don’t want your buildings to " fade in colour to the sky" then you can set the first fog distance beyond the total scale of your scene size.
You shouldnt get much of a performance hit either.
There are many ways of solving that, but all of them boil down to the same thing, put something there to hide it.
If you absolutely need your floor to be flat like that you can just make it much bigger.
You could use your own custom skysphere.
You could use fog.
You could use meshes like mountains (although you did say that you can’t do that, for whatever reason.)
You could increase the height of your terrain.
It all boils down to “hide it”.
finally i found someone who got the same problem as me 
what is your cliping distance far for the camera ? : i went at 2000 but my need would be 6000…
