ha-ha.

A ha-ha refers to the hidden deer fence that was a common feature of 18th-century English estates. Olin Partnership's (landscape architects) ha-ha for Washington Monument is an unobtrusive retaining wall that follows the paths that curve their way up the hill to the plaza, and is used as security from truck-bomb attack.
(courtesy of Witold Rybczynski in his article I Came, Eyesore, I Conquered at www.slate.com)