Most people have made this mistake of thinking Middle-earth is a particular kind of earth or is another planet of the science fiction sort but it's just an old fashioned word for this world we live in, as imagined surrounded by the Ocean.
J. R. R. TolkienGandalf: Confound it all, Samwise Gamgee. Have you been eavesdropping? Sam: I ain't been droppin' no eaves sir, honest. I was just cutting the grass under the window there, if you'll follow me. Gandalf: A little late for trimming the verge, don't you think? Sam: I heard raised voices. Gandalf: What did you hear? Speak. Sam: N-nothing important. That is, I heard a good deal about a ring, and a Dark Lord, and something about the end of the world, but... Please, Mr. Gandalf, sir, don't hurt me. Don't turn me into anything... unnatural.
J. R. R. TolkienTorment in the dark was the danger that I feared, and it did not hold me back. But I would not have come, had I known the danger of light and joy. Now I have taken my worst wound in this parting, even if I were to go this night straight to the Dark Lord. Alas for Gimli son of Glรณin!
J. R. R. Tolkien