Here I'll expain it another way so maybe it makes more sence. Never Never start something like this on a infinite plane ! It's basically flat and is difficult to work with. Start instead on a really huge flatish terrain. Problem is here you already did all this work on your mesh. So now your only option is to try to save your work by making the ground you got there bigger extending several miles off in the distance. Doing that is going to cover the hole so you'll need to crave that out with a cube. The cube can be bigger only it can't be bigger that your work here. Size of the cube is not critical here. Because the way it's grouped it will only cut out of the bigger terrain. Make sence now?