Understanding submesoscale features in the interior ocean and developing new approaches to observing interior Lagrangian circulation structures are the goals of the “Researching Interior Ocean Trajectories” Departmental Research Initiative (DRI) of the Office of Naval Research. Our overall goal is to better understand and model interior ocean trajectories and subsurface Lagrangian coherent structures by i) optimizing sensing plans for the most informative observations, ii) quantifying interior circulation patterns, water pathways, coherent sets, and submesoscale processes, iii) utilizing our probabilistic data-assimilative multiresolution (non)-hydrostatic ocean modeling capabilities, and iv) adapting models and analyses with machine and Bayesian learning of closures and parameterizations.