The Bay of Biscay formed during the Cretaceous as a consequence of the opening of the North Atlantic. The convergence between the Iberian and Eurasian plates during the Cenozoic resulted in its partial closure and the building of the Pyrenean-Cantabrian Mountains on land. Most of this deformation and shortening at the south-easternmost part of the Bay of Biscay concentrated in the North Iberian Margin, making this area a unique place to study the initial stages of deformation in a passive margin. In summer 2003, 11 deep seismic reflection profiles were acquired in the MARCONI seismic experiment, providing a new 3D image of the structure of the bay. This new dataset shows that the structure of this margin is characterized by a thick sequence of Mesozoic-Cenozoic sediments (up to 4 s TWT, up to 6 km) partially deformed by northward vergent thrusts and related folds. The interpretation includes three sedimentary sequences separated by unconformities that correspond to the pre-, syn- and post-tectonic units. A Mesozoic sedimentary basin imaged has indications of a Cenozoic tectonic inversion. Evidences of lateral and/or transfer structures coincide with two important N-S striking submarine canyons. The overall crustal structure of the south-easternmost part of the Bay of Biscay has been interpreted as a thinned continental or transitional crust underthrusted to the S below the extremely steep North Iberian continental slope.