It's a sunken ditch at the end of a garden - to prevent stock ie cattle, sheep etc wandering in to the garden insead of a wall or fence. You have the illusion that the garden continues out to the fields.
How about playing The Laughing Policeman by Charles Penrose? (Ha-ha-ha-ha-ha -ha -ha etc).
You could also mention the Terry Pratchett character 'Bloody Stupid' Johnson, who invented the ho-ho, a ha-ha with a ditch 20 feet deep. (The name came from the laughter of the owner when someone fell into it.)
Or a palindrome? "Ah! A ha-ha!"