The propagation of interfacial waves in free and constrained conditions, such as deep and shallow water, has been broadly studied over centuries. It is a common event that anyone can witness, while contemplating the ocean waves washing ashore. As a complementary configuration, this work introduces waves propagating on an interface restricted by its pinning to the solid microstructures of an underwater superhydrophobic surface. The latter has the ability to stabilize a well-defined microscale gas layer, called a plastron, trapped between the water and the solid phase. The acoustic radiation force produced with focused MHz ultrasound successfully triggers kHz "plastronic waves", i.e., capillary waves travelling on a plastron's gas-water interface. The exposed waves possess interesting features, i.e., (i) a high propagation speed up to 45 times faster than conventional deep water capillary waves of comparable wavelength and (ii) a relation of the propagation speed with the geometry of the microstructures. Based on this and on the observed variation of wave speed over time in conditions of gas-undersaturated or -supersaturated water, the usefulness of the plastronic waves for the non-destructive monitoring of the plastron's stability and the spontaneous air diffusion is eventually demonstrated.