The sinus (maxillary) is a naturally occurring space in the upper jaw. It is the size of a pea when you are born on the side of your nose, as the face grows, the sinus grows. Sometimes it takes up most of the space in the back of the upper jaw.
The sinus is a hole in the jaw, however the body can't see this hole because of the sinus lining. We only have to lift the sinus lining and place something underneath it to keep the space open, and the body will form bone as it does in an extraction socket. Nobody has discovered what it is there for..