  Geodynamic modelling and seismic studies have highlighted the possibility
that a thin layer of low seismic velocities, potentially molten, may sit atop
the core-mantle boundary but has thus far eluded detection. In this study we
employ normal modes, an independent data type to body waves, to assess the
visibility of a seismically slow layer atop the core-mantle boundary to normal
mode centre frequencies. Using forward modelling and a dataset of 353 normal
mode observations we find that some centre frequencies are sensitive to
one-dimensional kilometre-scale structure at the core-mantle boundary.
Furthermore, a global slow and dense layer 1 - 3 km thick is better-fitting
than no layer. The well-fitting parameter space is broad with a wide range of
possible seismic parameters, which precludes inferring a possible composition
or phase. Our methodology cannot uniquely detect a layer in the Earth but one
should be considered possible and accounted for in future studies.
