A ha-ha is a recessed landscape design element that creates a vertical barrier while preserving an uninterrupted view of the landscape beyond. The design includes a turfed incline that slopes downward to a sharply vertical face, typically a masonry retaining wall. Ha-has are used in landscape design to prevent access to a garden, for example by grazing livestock, without obstructing views. In security design, the element is used to deter vehicular access to a site while minimizing visual obstruction. The name "ha-ha" is thought to have stemmed from the exclamations of surprise by those coming across them, as the walls were intentionally designed so as not to be visible on the plane of the landscape.
This picture shows a ha-ha at Hopetoun House in West Lothian, Scotland, which keeps animals off the lawn while remaining unseen from the main house, visible here in the background. The wall disappears from view as it curves away to the left of the photograph.Photograph credit: Andrew Shiva