A ha-ha is a sunken wall, invisible until you’re nearly on top of it.  They became popular in the 18th century when attitudes to nature in Britain were changing.  As the land became increasingly tamed and methodically exploited, rigidly ornamental gardens fell out of fashion to be replaced by carefully designed “wild” landscapes.  A ha-ha would keep animals away from the house but not spoil the view, so the garden blended in with the countryside around.

The name is thought to stem from the expression of surprise of someone unexpectedly coming across one of these unobtrusive barriers.

The photo shows an effective ha-ha at Renishaw hall in Derbyshire (image by B-nik on Flickr).