how to delete specific pieces of terrain in unity

well, i was making my game and with the story you go up stairs and exit just at the surface of the terrain and since my stairs run onder the terrain and i don’t want to walk through it: is there a way to delete specific pieces of terrain wich in this case is the piece of terrain between my stairs and the room you enter? an other solution was making this pieve transparent and deleting its collider but its a very specific part so how do i do that?

You need to use voxels.

Here’s a tutorial on it: http://forum.unity3d.com/threads/tutorial-procedural-meshes-and-voxel-terrain-c.198651/

Here is a good premade one in the asset store:

Here is a free minecraft style one! :