The first to postulate the existence of convection currents underneath the earth's crust - and hence to formulate some of the basic ideas still used to explain the mechanism of continental drift - was the geologist Otto Ampferer. He saw convection currents mainly as a mechanism for mountain building. The first to formulate continental drift in terms of convection currents was Arthur Holmes. The concept was later expanded into the sea-floor-spreading hypothesis by Hess, Dietz, Wilson and others, and still the most widely used model to explain continental drift.