Light transmitted through a metal film that is perforated with holes smaller than the wavelength of light behaves in a surprising way: much more light is transmitted than initially expected for such small apertures. The reason for this behaviour has been a mystery, but physicists in France may now have found an explanation. Philippe Lalanne and colleagues of the Institut d'Optique in Palaiseau say that surface plasmon polaritons as well as quasi-cylindrical waves are responsible. The result, which challenges existing theories, could be important for designing new nano-optical devices.