Hydrodynamic surface waves propagating on a moving background flow experience
an effective curved space-time. We discuss experiments with gravity waves and
capillary-gravity waves in which we study hydrodynamic black/white-hole
horizons and the possibility of penetrating across them. Such possibility of
penetration is due to the interaction with an additional "blue" horizon, which
results from the inclusion of surface tension in the low-frequency gravity-wave
theory. This interaction leads to a dispersive cusp beyond which both horizons
completely disappear. We speculate the appearance of high-frequency
"superluminal" corrections to be a universal characteristic of analogue gravity
systems, and discuss their relevance for the trans-Planckian problem. We also
discuss the role of Airy interference in hybridising the incoming waves with
the flowing background (the effective spacetime) and blurring the position of
the black/white-hole horizon.Comment: 29 pages. Lecture Notes for the IX SIGRAV School on "Analogue
Gravity", Como (Italy), May 201