Most people think of low-amplitude ocean waves as being linear to a good approximation but it turns out that this is far from the case. Mark Ablowitz and Douglas Baldwin of the University of Colorado in Boulder report on nonlinear interactions that occur every day at low tide on two flat beaches at Nuevo Vallarta, Mexico, and Venice beach, California. Waves that intersect in X and Y shapes are closely related to interacting "line soliton" solutions of the nonlinear Kadomtsev–Petviashvili equation.

Their observations follow a tradition of soliton-spotting that began in 1834, when a naval architect, J S Russel, first recorded a solitary wave on the Union Canal in Edinburgh. They show that these nonlinear interactions are not rare events, as had been previously thought. Apart from being something to look out for while on holiday, such interactions play an important role in the formation of tsunamis – though, of course, with much larger waves.