Complex sentence: A popular version relates that a local resident named "Dick" built a water-powered grain mill in the 19th century; when in operation, the mill made a "thumping" sound that was heard throughout the surrounding forest.
Split into simpler sentences: A popular version relates that a local resident named "Dick" built a water-powered grain mill in the 19th century. When in operation, the mill made a "thumping" sound that was heard throughout the surrounding forest.