  The conversion of positive-frequency waves into negative-frequency waves at
the event horizon is the mechanism at the heart of the Hawking radiation of
black holes. In black-hole analogues, horizons are formed for waves propagating
in a medium against the current when and where the flow exceeds the wave
velocity. We report on the first direct observation of negative-frequency waves
converted from positive-frequency waves in a moving medium. The measured degree
of mode conversion is significantly higher than expected from theory.
