When a charged particle travels faster in a fluid than light in the same fluid, an ominous blue glow is produced, Cherenkov radiation. This radiation is emitted with a distinctive crossed structure in the wake of the charged particle. Similarly, when a duck swims at a critical speed across the surface of water, its wake propagates forward in a similar crossed fashion. Both forms of emitted waves travel with a crossed, X-shaped structure without any change in form. In hydrodynamics, this permanent form allows extremely large waves to be propagated without any change in their amplitude.
I'll discuss experiments undertaken during my PhD to create X-waves (and heart-shaped waves) in a circular basin and investigate their stability as amplitude is increased.