In the 1950s, mapping of the Earth’s ocean floors showed the Mid-Atlantic Ridge to have valleys and ridges. Its central valley is seismologically active and the epicentre of many earthquakes. Ewing and Heezen discovered the ridge to be part of a 40,000-km-long essentially continuous system of mid-ocean ridges on the floors of all the Earth’s oceans. 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 as plate tectonics.