  The eigenfrequencies of nonradial oscillations are a powerful probe of a
star's interior structure. This is especially true when there exist
discontinuities such as at the neutron star (NS) ocean/crust boundary, as first
noted by McDermott, Van Horn & Hansen. The interface mode associated with this
boundary has subsequently been neglected in studies of stellar nonradial
oscillations. We revisit this mode, investigating its properties both
analytically and numerically for a simple NS envelope model. We find that it
acts like a shallow surface ocean wave, but with a large radial displacement at
the ocean/crust boundary due to flexing of the crust with shear modulus $\mu\ll
P$, the pressure. This displacement lowers the mode's frequency by a factor of
$\sim(\mu/P)^{1/2}\sim0.1$ in comparison to a shallow surface wave frequency on
a hard surface. The interface mode may be excited on accreting or bursting NSs
and future work on nonradial oscillations should consider this mode. Our work
also implies an additional mode on massive and/or cold white dwarfs with
crystalline cores, which may have a frequency between the f-mode and g-modes,
an otherwise empty part of the frequency domain.
