One of the more mysterious structures in the United States, the Newport Tower is a circular stone building in Newport, Rhode Island. The most believable (but least interesting) theory is that the tower was originally the base of a windmill built in the 1600s or 1700s by some of America's first European settlers. However, some people hypothesize that it is several hundred years older than commonly thought and provides evidence that someone other than Columbus made the first landing in the New World.
Carbon dating of the foundation and excavations of the surrounding areas seem to support the windmill hypothesis. However, there is also speculation that the tower was some sort of observatory because its windows align with various stars and moon positions. There are also windows that align with the sun's position during the summer solstice. These odd features have led to theories about Vikings, Chinese sailors, and even the Knight Templars being responsible for the construction of the tower.