Ear of Dionysius, Sicily, Italy
This large limestone cave has a great sound legend attached to it. The story goes that the tyrant Dionysius (ca. 432–367 BC) used this place as a prison. The wedge shape of the cavern causes peoples’ conversations to be focussed and amplified at the roof of the cave, 22m above the floor. Supposedly this enabled guards to spy on prisoners by listening to the amplified sound through a small hidden opening at the top of the cave: even when the prisoners spoke in whispers.
“The tearing of a piece of paper makes a noise not unlike that occasioned by knocking a heavy stick against a stone” Conrad Malte-Brun, 1829.
Unfortunately, it’s no longer possible to hear the effect because of safety fears; in the past travellers were hosted up by rope and pulley to the opening. Consequently, a modern listener is just left to enjoy the reverberance at ground level, marvel at the legend and take in the cavern’s ear-like shape.
Gino Iannace and collaborators  have recently been able to make measurements and test whether the spying myth is true. Rather disappointing, they found that any whispered conversations were unintelligible, lost in a blur of reverberation. The sound sample at the top of the page was reconstructed from their measurements, even with carefully enunciated speech, it is hard to understand what is being said.
The Parco Archeologico della Neapolis, which includes the cavern, also has a Greek amphitheatre which sound tourists can admire.
This video has good sound, but for some reason has been shot on its side.
Credits and sources
Site suggested by Nick Antonio
 G. Iannace, L. Marletta, F. Sicurella and E. Ianniello, “Acoustic measurements in the Ear of Dionysius at Syracuse (Italy)” Internoise 2010.
Photo: Michael Wilson (c) some rights reserved