A haha is a boundary to a garden or park, usually a buried wall or shallow ditch designed not to be seen until closely approached.
I’m told there’s a rather nice haha at Elvaston Castle just outside Derby. From the house there appears to be an unobstructed vista into the distance, despite the presence of the main road to Derby crossing the field of view about 200 yards away. Unfortunately, when the house was designed, they hadn’t invented double-decker buses or lorries, so the effect is a bit spoilt by the sudden appearance of the top half of a bus going past from time to time.