Walls that disappear into the landscape.

A ha-ha (French: hâ-hâ or saut de loup), 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 while preserving an uninterrupted view of the landscape beyond.

The function of this ditch is usually to keep animals away from the house. One could view the ha-ha simply as a wall integrated into its surroundings, or as a device that removes the visible barrier between nature and mankind.