Summary PROTEUS Plumbing Reservoirs Of The Earth Under Santorini “One must wait until the evening, to see how splendid the day has been.” Sophocles On this research expedition the R/V Marcus G. Langseth leg MGL1521 collected dense seismic data at Santorini volcano in the eastern Mediterranean, Aegean Sea.  The study was supported by the National Science Foundation Grant number (1548026)*.  During the 26-day expedition, 91 four-component, ocean bottom seismometers (OBSs) were throughout the area to record seismic energy from the 36-element, 6600 cu. in. airgun array of the R/V Marcus G. Langseth.  The data will be used for 3D, anisotropic seismic tomography and full seismic waveform inversion.   The goal of the research project is to examine the entire crustal magma plumbing system beneath an arc volcano. The magma geometry and connections throughout the crust are physical parameters that control magma migration, storage, and eruption and are thus important to understand geohazards. More broadly, this study will help answer questions about how the processing of magma at arc volcanoes forms the rock compositions that dominate the lower continental crust. The ocean bottom receivers were from the NSF-supported Ocean Bottom Seismograph Instrument Pool (OBSIP); 61 and 30 OBSs were provided by the Scripps Institution of Oceanography (SIO) and the Woods Hole Oceanographic Institution (WHOI) OBSIP groups, respectively.   Of the 91 OBS 90 were successfully recovered.  By site, the data return rate appears to be somewhat lower than is typical for active source short-period deployments.  The seismic source throughout the experiment was the Langseth’s 36-gun array, with a total volume 6600 cu. in.  Airgun data were collected along 2500 km of track line an average shot spacing of 144 to 165 m.  In addition to the seismic data, swath bathymetry, gravity and magnetics data were collected throughout the region. The structure of this report is as follows:  The sections prior to the appendices briefly summarize the scientific and operational objectives, the events that transpired during the cruise, and the overall quality and characteristics of the seismic data.  The remainder of the report contained in the appendices is primarily of a technical nature and useful to someone working with the PROTEUS data.   *Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.