The results of recent studies of elastic wave propagation within the earth are reviewed and different interpretations of the earth's interior which have been proposed, particularly in recent American works, are outlined. The results of these studies and of oceanographic work are incompatible with certain classic theories. The main facts are the differences between the continental and oceanic crust and the relatively recent (Tertiary and Quaternary) age of the great zones of continental subsidence, where a transformation from a continental to an oceanic-type crust is implied. The concept of permanent ocean basins is rejected. Differences between the continental and oceanic crust are considered essentially a function of physical state rather than petrographic composition. Continental zones that have subsided are characterized by greater seismic velocities because of the extra pressure to which they have been subjected in the course of the ages.