Go to Modify panel and add an Extrude modifier to Ground_Cutout which will be used to make the whole into the ground. Set the Extrude amount = 10.
Then in the Front viewport, move it to the down a little so that it intersetcs the Plane object.
In the Create panel > Geometry, click the downward arrow button for choosing the category (next to the Standard Primitives). Choose Compound Objects from the drop-down list.
