You’ve seen it on maps before, but have you ever thought about how weird the northern border of the U.S. state of Delaware actually is? Most state boundaries follow rivers (like the Connecticut River between Vermont and New Hampshire) or mountain ranges (the Bitterroots between Idaho and Montana) or boring straight lines (all four sides of Wyoming). But the top of Delaware is unique in America: a seemingly perfect circle carved out of southern Pennsylvania. Why a circle?

View Twelve-Mile Circle in a larger map

In 1682, when the Duke of York originally granted Delaware to William Penn, he defined its top bit as “all that the Towne of Newcastle otherwise called Delaware and All that Tract of Land lying within the Compass or Circle of 12 Miles about the same scituate_ (sic)_.” In other words, Delaware was defined as a twelve-mile radius around New Castle. And it’s not just on the northern border, either. It might be too small to see in a road atlas, but there’s a small stretch of the Delaware-Maryland border, less than a mile long, that bulges west slightly due to the twelve-mile arc intersecting the boundary line there.

The border was first surveyed in 1710, but different sections of the arc have been drawn at different times, using a variety of centers. (The cupola of the New Castle courthouse is the theoretical center.) So Twelve-Mile Circle isn’t a perfect circle at all, but a series of different arcs that have been smoothed into one, more or less.

The Duke of York’s 1682 grant explicitly defines the arc as continuing through “the River Delaware in America and all islands in the same River” instead of to the middle of Delaware River, as is more common. This has led to centuries of legal squabbling with New Jersey, the eastern neighbor that doesn’t, apparently, own any of the river on Twelve-Mile Circle. Delaware’s case has been upheld by the Supreme Court three times, as recently as 2007, when New Jersey wanted to put a natural gas terminal on the river’s edge. Delaware even considered a bill calling on the National Guard to protect its borders from the threat of New Jerseyan invasion.

There are a few dozen international boundaries drawn with arcs as well, though none so large or beautiful as Delaware’s. Zimbabwe’s Tuli Safari Area is a half-circle that bulges eight miles across the Shashi river into Botswana. My favorite round border is the colonnade around the Vatican’s Piazza San Pietro. Part of that wall is the Vatican’s eastern border with Rome, and the great Italian sculptor Gian Lorenzo Bernini laid out the piazza as a vast ellipse, or oval, when he redesigned it in the 1660s. So now the national border is a big oval as well.

Explore the world's oddities every week on CondeNastTraveler.com with Ken Jennings. Check out his latest book, Maphead__.