A primary feature of ocean acoustic propagation is that vertical environmental scales and gradients are generally an order of magnitude smaller than horizontal scales. As such, ocean acoustic experiments and modeling have developed with the view that the in-plane propagation is primary. There are environments, however, where the horizontal variability can be significant enough to lead to a change in the observed acoustic field due to out-of-plane refraction or diffraction. This is particularly evident at very low frequencies, where the open ocean can no longer be considered deep water and the seafloor topography is not negligible. In this paper, several recent observations of the impact of three-dimensional propagation on global propagation will be presented, as well as a fully 3-D parabolic equation, which is efficient enough to handle very long range propagation at low frequencies. The final modeling illustration will be the impact of ocean mesoscale on arrival angle of low frequency long range signals. This is implications to the detection capability of systems like the Comprehensive Testban Treaty Organization.