Airplanes are loud. Very loud. And all that means is that they're pumping out massive amounts of acoustic energy that's being wasted assaulting your eardrums. Airport runways made of custom "designer" gravel could harvest some of this energy and turn it into useful amounts of electricity.
As it is, materials such as runway concrete already absorb substantial amounts of acoustic and vibration energy when aircraft are powering up for takeoff. The reason we can't harvest it directly is because this energy is all over the place, and there's no way to efficiently channel it in a way that would turn it into a useful amount of electricity.
Researchers at the University of Buffalo have proposed restructuring runway concrete with what they're calling "designer gravel," which is made up of particles whose shapes have been specifically designed to channel energy into usable bursts. So, where irregularly shaped or round gravel particles just pass packets of acoustic energy along, these new particles could have their shapes tuned to condense and focus energy into useful pulses and then send those pulses to a collection point where it can be harvested.
At this point, the designer gravel is more of a mathematical proof than a physical one, but as nanotechnology evolves from a tech buzzword into an everyday manufacturing process, even rocks are going to be engineered to be smarter and more efficient.