myrddin173
Maester
Tolkein didn't invent Middle Earth, he borrowed it from norse mythology. If you look at one of the old covers, the Hobbit cover has runes all over it. A lot of his story was inspired by norse myth, so I don't think it would be at all plagiarism. The Norse believed there were many levels to the world, and the middle one, Midgard (Middle Garden), was where people lived.
He may not have come up with the name but most of the world his his creation, though there are some distinct Norse influences he was actually working to come up with a British mythology.