sentences:  Modern archaeological studies have indicated the area was inhabited through the 1920s.By the late 20th century, all vestiges of New Philadelphia had vanished save fragments of glass and pottery, and traces of the town's gravel streets.The town site was added to the U.S. National Register of Historic Places on August 11, 2005; subsequently, New Philadelphia Town Site was designated a National Historic Landmark on January 16, 2009 because of the significance of its history and archaeology.