In many areas of the world, the continental lithospheric upper mantle contains bright, continuous, regionally extensive, seismic reflectors. Despite the increasingly common observation of such mantle reflectors on deep seismic reflection profiles, their significance and geologic origin remain obscure. We report results from a series of seismic experiments acquired across two of the brightest of these reflectors and provide new constraints upon the composition and thickness of the reflecting region. These new seismic data reveal regionally extensive dipping and subhorizonal slabs of high-velocity (>8.4 km/s), high-density (>3500 kg/m3) material, several kilometres (>2 km) thick, entrained within otherwise unremarkable upper mantle. The geometry, physical properties, and geologic setting of these mantle reflectors suggest that they represent fragments of eclogitic oceanic crust—a relict of pre-Caledonian oceanic subduction now preserved within the lower continental lithosphere. Such relict subduction zones appear to be widespread within the continental lithosphere and to exert an important influence upon the location and style of subsequent continental deformation.