Three-dimensional seismic data reveal a series of kilometer-wavelength hummocks and intervening depressions that formed above a 22-km-long section of a Pliocene–Pleistocene deep-water slope channel on the western margin of the Niger Delta. The depressions are 600–900 m in diameter and 30–50 m deep; the intervening convex-upward, symmetrical hummocks are 200–1500 m wide. The hummock-depression structures are interpreted to be new, large-scale types of soft-sediment deformation phenomena that formed as a result of fluidized sediment flow that was initiated because of overburden seal failure during differential loading of sand at the early stages of burial. Fluidized sediment was expelled onto the contemporaneous seabed, some of which was preserved within depressions that formed owing to the sediment removal at depth and the resultant collapse of the overburden. Similar structures are commonly recognized at a centimeter to meter scale, but their occurrence on a much larger kilometer scale has, until now, been only a theoretical possibility.