The Ear of Dionysus is a large artificial cavity thirty metres high and forty metres deep. It is so called both because from the outside the cavity takes the shape of an ear, and because it is linked to an ancient Syracusan legend: the tyrant Dionysus had prisoners locked up inside the cavity, as the acoustics of the place allowed him to eavesdrop on everything that went on inside. The result was the death sentence for those who said anything untoward and for conspirators.
In fact, the acoustics in the Ear of Dionysus are very special, prompting visitors to sing songs inside.