Ever been out on a lake and heard the whispers of someone far away on shore? A quirk of atmosphere and a quirk of physics work together to turn the body of water into a telephone.
Imagine you are driving in a car down a long, open stretch of plain. You keep moving and moving in a straight line, until suddenly, the tires on the left side of your car are moving faster than the tires on the right side of your car. That means the right side drags, while the left side zooms ahead. You start to turn to the right. If you were on a road, you'd be in trouble. Since you're on a plain, the car drifts right, right, right, until you describe a kind of arc, like a rainbow, and start driving back the way you came.
Now imagine you're a sound wave.
That might be a harder to do, but the basic motion is the same. When a sound wave travels from cold air to warm air, the side that hits the warmer air goes faster than the side in the cool air. As that side accelerates, it curves the path of the overall wave until it forms an arc and comes back down to Earth.
Where does this happen? It could happen nearly anywhere, but it's generally most noticeable on still lakes on summer evenings. As the sun goes down the lake gives up its heat, and the warm air rises high into the atmosphere. Cold air gathers underneath, and so sound coming up from the lake, or from the shore, will hit that warm air and curve back. There are no buildings or trees above a lake to interfere with the sound, and it's quiet out on the water. People rowing on the lake will sometimes hit the spot where the sound come back down, and will be able to clearly hear conversations that are taking place a long way away. The lake forms a kind of huge, impractical telephone, transporting sound from one specific place to another.
[Via Good Vibrations]