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A ha-ha is a recessed landscape design element that creates a vertical barrier while preserving an uninterrupted view of the landscape beyond.
The design includes a turfed incline which slopes downward to a sharply vertical face (typically a masonry retaining wall). Ha-has are used in landscape design to prevent access to a garden, for example by grazing livestock, without obstructing views. In security design, the element is used to deter vehicular access to a site while minimizing visual obstruction.
The name "ha-ha" is thought to have stemmed from the exclamations of surprise by those coming across them, as the walls were intentionally designed to be invisible.
Bit of a strange one, 2 years ago I set up a DS1817 HA cluster, to provide AD for a small subsiduary of the company I work for. All of a sudden the cluster has started to hand out dhcp leases, despite the fact that the dhcp server package is not installed and the dhcp server page is...