ha-ha n. a ditch with a wall on its inner side below ground level, forming a boundary to a park or garden without interrupting the view.  Oxford Dictionary

As in:

"Fanny, feeling all this to be wrong (of course!) couldn't help making an effort to prevent it. "You will hurt yourself, Miss Bertram," she cried, "you will certainly hurt yourself against those spikes - you will tear your gown - you will be in danger of slipping into the ha-ha. You had better not go."
                                                                                                            Mansfield Park, Jane Austen