If you drive past the Adelaide Hills, you’ll enter the Barossa Valley, an area famous for its wines. Besides the many winery tours you can do in the area, there is a big dam, aka the whispering wall.
It is a gigantic structure that holds the water of the Barossa reservoir close to Williamstown. The dam crosses about 100m and you can walk all the way across. What they didn’t know when they built it was that the acoustics of the dam were perfect.
When you stand on one end of the dam and whisper a secret message, anyone on the other end can hear you clearly. This is possible because of the curve in the design of the dam. Entry to the dam is free.