mountain crossing problem

Hello everyone, sorry for my bad English
I have the following problem I created using blender starting from a raw image of a mountain. The problem is that when I hand it to unity and sorvolp with a kind of helicopter, the various tops no resistance as there were not I tried to apply a colider mesh without solving the problem.
Thanks for any advice

Do you have Mesh collider? Unity - Manual: Mesh collider component reference Does helicopter have a RigidBody? All colliders must have IsTrigger = false.