  Surface waves in classical fluids experience a rich array of black/white hole
horizon effects. The dispersion relation depends on the characteristics of the
fluid as well as on the fluid depth and the wavelength regime. We focus on the
shallow-water regime, and discuss the experimental proof that the circular
hydraulic jump marks the transition between a supercritical and a subcritical
flow regime. This finally confirms a theoretical conjecture formulated by Lord
Rayleigh nearly 100 years ago. It also confirms that the circular jump
corresponds to the spontaneous formation of a hydrodynamic white hole, with
interesting characteristics from the point of view of analogue gravity. We
study the dispersive regime, mention some lessons about the trans-Planckian
issue and describe possible directions for future work.
