For nearly two years, over the course of 1791 and '92, men surveyed the length of the original border of the District of Columbia, the not-yet-developed capital city of the United States, and placed special marker stones at 40 points along the diamond. Many of these stones still exist at or near their original locations.
The Boundary Stones site (which I learned of via metafilter) is an interactive map of each stone's site. "The boundary stones are the oldest federal monuments," the site notes. And even 216 years and enormous area development (and stone damage) later, they're still there.