The Old Stone Mill, also called the Newport Tower, stands in Touro Park. It is about 28 feet (8.5 m) tall and about 22 to 23 feet across.
For a long time people argued about who built it, and some guessed it was Viking. Wikipedia says carbon-14 testing indicates it is the remains of a 17th-century windmill.
A document from 1741 already calls it 'the old stone mill'. It is a good example of an old mystery with a well-tested answer.