Results from the first testing of the R/V Marcus Langseth as a platform for collecting seismic reflection data from the water column of the ocean demonstrate that large-amplitude lee waves can be acoustically mapped. A seismic profile collected in the Caribbean Sea offshore Costa Rica shows disturbances in finestructure, which we interpret to be lee waves, propagating hundreds of meters vertically through the water column above seafloor ridges. Waves show vertical displacements of 30–50 m and horizontal wavelengths of 300–3000 m. Reflector displacement spectra calculated in the region containing the lee waves exceed Garrett-Munk energy levels by up to a factor of 10 at horizontal wavelengths of 300–3000 m, suggesting a locally derived source of internal wave energy consistent with our interpretation. Our results show that it is possible to image large-scale lee waves, a phenomenon potentially responsible for dissipation and mixing within the ocean.