In the 1950s, mapping of the Earth’s ocean floors by Bruce Heezen, Maurice Ewing, Marie Tharp and others revealed that the Mid-Atlantic Ridge had a strange bathymetry of valleys and ridges,[6] with its central valley being seismologically active and the epicenter of manyearthquakes.[7][8] Ewing, Heezen and Tharp discovered that the ridge is part of a 40,000-km-long essentially continuous system of mid-ocean ridges on the floors of all the Earth’s oceans.[9] The discovery of this worldwide ridge system led to the theory of seafloor spreading and general acceptance of Wegener's theory of continental drift and expansion in the modified form of plate tectonics. The ridge is central to the breakup of the hypothetical supercontinent of Pangaea that began some 180 million years ago.