Previous analyses of the variation of depth with age in oceanic basins have demonstrated a systematic square root of age dependence in crust younger than 80 Ma. These studies used depths determined from marine seismic data, which have uncertainties due to the assumed sediment velocity. As an alternative, we compiled the basement depths from all DSDP and ODP drill sites that sampled 'normal' ocean crust in the Atlantic, Pacific and Indian Oceans, up to recent drilling (Leg 136). We applied further criteria to the data, rejecting any site not formed at a mid ocean ridge, or that did not penetrate extrusive basalts of MORB composition. The remaining high quality data set is small (77 sites), but adequate to test models of the thermal structure of the upper mantle. Our new results also show that the square root of age (t1/2) variation incorrectly estimates the observed depth for crustal ages older than 90 Ma. However, our data suggest a plate thickness of 104¿9 km and a basal temperature of 1400¿140 ¿C, and agree, at all ages, with the recent inversion by Stein and Stein, 1992. ¿ American Geophysical Union 1992