  Recently, the topological physics in acoustics has been attracting much
attention. However, all the studies are aimed to elastic or airborne sound
systems. Realizing topological insulators for underwater sound is of great
importance, since water is another crucial sound medium in addition to solid
and air. Here we report an experimental study on the valley-projected edge
states for underwater sound. The edge states are directly observed in our
ultrasound scanning experiments, together with a solid evidence for the
valley-selective excitation. The experimental data agree well with our
numerical results. Prospective applications can be anticipated, such as for
underwater sound signal processing and ocean noise control.
