During Integrated Ocean Drilling Program (IODP) Expedition 340T we conducted borehole logging at Atlantis Massif, an oceanic core complex (OCC) just west of the spreading axis of the Mid-Atlantic Ridge, 30°N. Seismic data for the 800–1400 m interval cored in Hole U1309D during IODP Expeditions 304 and 305 are the primary new information, whereas a number of ancillary measurements at the site also aim to address the question of where seawater penetrates and alters young oceanic lithosphere, thus hydrating a key chemical reservoir in the Earth. The detachment fault that controlled formation of Atlantis Massif (Cann et al., 1997; Blackman et al., 2002; Schroeder and John, 2004; Karson et al., 2006) is known to have localized fluid flow in a ~100 m thick zone of deformation/alteration that is exposed at the seafloor (Boschi et al., 2006; McCaig et al., 2010). Additional zones of (at least) past seawater circulation are indicated in deeper known or inferred fault zones or portions of olivine-rich troctolite intervals that are altered to serpentinite (Hirose and Hayman, 2008; Michibayashi et al., 2008; Beard et al., 2009). Seismic reflectivity observed in multichannel seismic (MCS) data throughout the Central Dome and Southern Ridge of the massif (Canales et al., 2004; Singh et al., 2004; Blackman et al., 2009) may correspond to interfaces between zones of past and/or present fluid flow and the surrounding rock. The main goal of Expedition 340T was to begin to test this hypothesis.