The Unhappy Cannonballs are an invention of Dr. Zalost for his Unhappy Cannonball Project. These black cannonballs make people and animals extremely unhappy and lethargic upon contact. Interestingly, they have no effect on Eustace because he is immune to emotions. After being hit with an Unhappy Cannonball, the victim's skin turns green. The remedy to Unhappy Cannonballs are Muriel's Happy Plums.
Unhappy Cannonballs are made in a machine devised by Dr. Zalost. Upon adding Happy Plums into the giant bowl that produces the Unhappy Cannonballs, the machine starts making pink cannonballs, which undo the effects of the Unhappy Cannonballs. Being hit a total of three times by an Unhappy Cannonball seemingly petrifies a target.