This happens because the landscape object is just a plane distorted along the Y-axis. It has no thickness so part of the cube pokes through it and is unaffected by the boolean. To do what you want, select all the polygons in the landscape then either use the make thicker plugin, or in R9 do extrude with 'create caps' set. Once your landscape has some thickness make sure the area of the cube to be cut away is totally enclosed within it and the boolean will work as intended.

Steve