A ha-ha is a fence which is concealed in a trench, allowing for the creation of a barrier without the interruption of the view. Ha-has were developed in England in the 1700s, as part of the Landscape Gardening school, which valued long, sweeping views which would appear uninterrupted from most angles. Some examples of extant ha-has dating back to the 1700s can be found in England, and they have been incorporated into landscape design in other regions of the world as well.
When a ha-ha is constructed, a deep trench is dug. One side of the trench has a relatively straight side which may be reinforced with stone or wood, creating a fence, while the other side has a gentle slope. For someone walking across a lawn or stretch of land, the ha-ha will appear invisible until the walker reaches the trench. In some cases, a double ha-ha will be built, with a fence in the middle of a trench, acting as a barrier to people or animals approaching from either side.
For members of the Landscape Gardening school, the ha-ha fulfilled a very useful function. Most estates maintained farm animals like cattle, sheep, and horses, and these animals could destroy a formal garden or lawn in very short order. Prior to the development of the ha-ha, large and sometimes unsightly fences and walls were required to keep animals out of the formal gardens. After the ha-ha concept was conceived, gardens could have uninterrupted views across the estate, and animals could be effectively kept out.
In addition to being used to keep animals out, a ha-ha can also be used to camouflage or gracefully conceal fencing in other locations. Some cities, for example, use ha-has around buildings which require extra security, giving visitors the illusion of a view of sweeping lawns while effectively controlling access to buildings such as courthouses, jails, and city halls. Ha-has were also a common feature in the design of asylums in England in the 18th and 19th centuries, keeping people confined without offending the delicate sensitivities of the public.
While the ha-ha is classically a feature of the antique garden, there's no reason a ha-ha cannot be included in modern landscaping. Many people value big views, and a ha-ha is ideal for providing views and security. While some labor is required for installation, a ha-ha can last for hundreds of years if well-maintained, and it can dramatically change the look of a landscape.
Incidentally, the term “ha-ha” is said to be derived from the sounds of surprise made by people when they stumble across a ha-ha. In some landscaping manuals dating to the 1700s, the sunken fence is also described as an “ah-ah.”