I have a concrete patio in my back yard. The patio has an opening (5 feet x 3 feet) in it. The opening is about 3 to 4 feet deep and runs further inside about 10 feet under the patio. Please see the pictures attached. I am in the process of filling this hole up, but before I do that I just want to know why this hole and opening was made in the first place. I don't want to fill the hole and then realize that I shouldn't have done that.