BACKGROUND - Map in proces. will be surrounded by cliffs.
tunnel that has a ladder leading to the top of these cliffs (for sniping). ive made the top of the cliffs with simple terrain mesh (15x15) + a tunnel leading to top with ladder inside.

PROBLEM - I cant cut the mesh, so the tunnel goes through the mesh, cant go down tunnel! is there a easy way to workaround this? or will i have to have 4 different meshes around the tunnel??

thank you in advance!

In short yes, you cannot have a hole in your terrain mesh. However what you will need to do is split your existing meshes into smaller ones until you can just delete the piece that is blocking your tunnel.

Just select the mesh, show the vertices with the "V" key, shift+left-click on one that is in the row or column that you need to split the mesh on, then press Shift+Ctrl+"X" to split the mesh.

Once you do that you'll have two different meshes. Continue in that fashion and you should have the opening for your tunnel without completely redoing your terrain fine.

Awsome, thank you!