Because of their large wavelengths, VLF radio waves can diffract around large obstacles and so are not blocked by mountain ranges, and can propagate as ground waves following the curvature of the Earth.
VLF waves can penetrate seawater to a depth of at least 10 to 40 meters (30 to 130 feet), depending on the frequency employed and the salinity of the water, so they are used to communicate with submarines.