Ocean bootom seismographs operating in the median valley of the mid-Atlantic ridge have recorded both P and S waves from locally occurring earthquakes. In the Famous area a velocity ratio Vp/Vs=2.08 has been observed. This unusually high value is believed to reflect the high fissure porosity near the axis of the ridge. So far it has only proved possible to develop crude P and S velocity structures, but it is expected that more sophisticated models will soon be available. The ratio Vp/Vs as a function of the depth will then be a useful indicator of rock porosity beneath the median vallye floor and may show the depth to which hydrothermal circulations extends.