Scientists have drilled only about 1.5 km into the 6- to 7-kilometer-thick oceanic crust, so their understanding of oceanic crust comes largely from comparing ophiolite structure to seismic soundings of "in situ" oceanic crust. Oceanic crust has a layered velocity structure that implies a layered rock series similar to that listed above. In detail there are problems, with many ophiolites exhibiting thinner accumulations of igneous rock than are inferred for oceanic crust. Another problem relating oceanic crust and ophiolites is that the thick gabbro layer of ophiolites calls for large magma chambers beneath mid-ocean ridges. Seismic sounding of mid-ocean ridges has revealed only a few magma chambers beneath ridges, and these are quite thin. A few deep drill holes into oceanic crust have intercepted gabbro, but it is not layered like ophiolite gabbro.