We are collecting seismic data in the northeast Atlantic Ocean west of Spain to image faults under the seafloor that were involved in continental rifting and breakup and the initial opening of the Atlantic Ocean ~125 million years ago. This rift is notable because very little volcanism appears to have accompanied rifting, as observed in many other rifts worldwide. Instead, continental breakup here appears to have been sufficiently slow and cold that the rocks from the Earth’s mantle were exposed at the seafloor after the crust broke. Previous seismic imaging studies have revealed a continuous sub-horizontal structure that lies at the base of a series of fault blocks beneath the seafloor (called the “S reflector”), and there is significant controversy here and for similar features elsewhere on the role of this feature in accommodating extension and exposing mantle at the seafloor. This program involves collecting and analyzing a suite of geophysical data to image this structure, the overlying fault blocks, the exposed mantle and the sediments in 3D to reconstruct the evolution of the rift. We will collect 3D seismic reflection data to image faults and sediments during a 45-day cruise aboard the R/V Marcus G. Langseth, and three cruises on the F.S. Poseidon to deploy and recover ocean bottom seismometers that will be used to determine the velocity that sound travels through different layers of the earth, which yields information on their compositions.