Five deep crustal holes were drilled into the Mid-Atlantic Ridge near 37¿N by Glomar Challenger on Leg 37 of the Deep Sea Drilling Project. The drilling results and laboratory measurements of seismic velocity on samples from one hole of over 500 m into basement have shown that the low velocity upper crustal layer 2a consists of interlayered solid basalt and shattered and fractured volcanic material with extensive large scale porosity and voids and some inrercalated sediments. Gabbros and serpentinites with velocity appropriate for the lower crust were recovered from another hole.