For the first time the method of seismic oceanography was applied to identify fine structure of a water mass in greater depths (> 1500 m) close to the seafloor. The pathway of the upper high-velocity Western Boundary Undercurrent (WBUC) branch was tracked over the Eirik Drift, 200 km south of Greenland at seafloor depths between ~2200 and 3000 m. It appears as an upward convex structure attached to the slope with a transparent, i.e. well mixed, core surrounded by higher amplitude reflections. These reflect gradients and fine structure. Fine structure is a result of enhanced mixing processes, presumably due to entrainment of surrounding water of less momentum by the intensified deep current core. We show that this new information about structure and pathways of the WBUC could not have been gained by conventional oceanographic measurements alone.