  Sound transmission through water-air interface is normally weak because of a
strong mass density contrast. Here we show that the transparency of the
interface increases dramatically at low frequencies. Rather counterintuitively,
almost all acoustic energy emitted by a sufficiently shallow monopole source
under water is predicted to be radiated into atmosphere. Physically, increased
transparency at lower frequencies is due to the increasing role of
inhomogeneous waves and a destructive interference of direct and
surface-reflected waves under water. The phenomenon of anomalous transparency
has significant implications for acoustic communication across the water-air
interface, generation of ambient noise, and detection of underwater explosions.
