how could i put in a cave in my terrain

I am having problems putting in a cave because you can’t enter the cave because the terrain gets in the way. I am using personal version and I have a good cave pack but all I need to know is how to put a hole I my terrain but the place I want to put it is on a slope so how could a experienced unity user like me do it?

When terrain is selected, choose second option of terrain editing (Paint Height). There is a “Height” slider. Adjust it as you want and click option called “Flatten”. Terrain will goes up and you can make holes selecting “Raise / Lower Terrain” option and clicking on terrain holding Shift key.