I need to hide specific areas of terrain because I want to place a terrain object over a tunnel like this:
Any suggestion or trick?
There is no simple way to cut through terrain. You can use some shader trickery to cut visible holes in it, then use locational programing to say “it’s okay to go through the terrain in this region”, but that is very complex.
It’s far easier to just build the object that contains the tunnel (this is exactly how Elder Scrolls series of games handle tunnels), then plant that object on the terrain and hide the seam with lighting or bushes or what have you.