The sunk fence is a divider dug into the earth rather than one erected upon it, a ditch with one sloped side and a single retaining wall. This inverted concept allows for separation—between mown ground and grazed grass, for instance, between lawn and ﬁeld—without the use of vertical ﬁxtures or usual fencing materials such as wood, stone, or metal. Imported from landscaped parks in England by Frederick Law Olmsted in the mid–nineteenth century, the sunk fence was adapted to suit peculiarly American needs. In Olmsted’s design of Central Park, sunken roadways perform the same function as the sunk fence, entrenching what otherwise would interrupt the clean, aesthetic expanse of park or garden. Sunk fences are also called ha-has—a name suggesting both the startled exclamation of the person who stumbles into one, and the amusement of the witness to such antics.