Seismic data captured by ocean-bottom instruments have revealed "fossilized" traces of mantle flow beneath the Pacific Ocean's oldest preserved piece of crust. The patterns of ancient flow traced in the uppermost mantle are complicated, tracking changes in tectonic plate motion over time as well as mantle deformation around features such as a sunken remnant of the lithosphere (the crust and the uppermost brittle mantle) and upwelling hotspots.