ha-ha

thumb|Comparison of a ha-ha (top) and a regular wall (bottom). Both walls prevent access, but one does not block the view looking outward.

A ha-ha ( or ), also known as a sunk fence, blind fence, ditch and fence, deer wall, or foss, is a recessed landscape design element that creates a vertical barrier (particularly on one side) while preserving an uninterrupted view of the landscape beyond from the other side. The name comes from viewers' surprise when seeing the construction.