A visiting preacher at the church I currently attend gave a very interesting verse by verse exposition of Psalm 139. When he came to the above he stated that the lowest part of the earth meant exactly that. He cited that there are a maximum number of proteins needed for the human body to be complete and for the building blocks (amino acids) to form, and part of these proteins come from the plant life around us, which are consumed by human parents, without which no human body can be formed. Well, I am not quoting him verbatim but I think I have captured enough of the gist of what he said. I would be very interested to hear what others have to say. Now, of course, the above verse wasn't the focus of what he preached on, rather, that every verse of the psalm led to David's final plea to be examined by God thoroughly.