The research cruise SO-240 took place from May 3rd, 2015 until June 16th, 2015 at about 12°N and 118°W, approximately 900 nm offshore Mexico in the equatorial NE Pacific. This cruise started and ended in the port of Manzanillo on the west coast of Mexico. Scientists from the Federal Institute for Geosciences and Natural Resources (BGR), the University of Bremen, the Jacobs University Bremen, the Alfred Wegener Institute Helmholtz Center for Polar and Marine Research (AWI), Bremerhaven, Germany (AWI), and the German Centre for Marine Biodiversity Research Wilhelmshaven participated in the cruise. The objective of this cruise was to investigate the regional pattern of seawater circulation within the basaltic seafloor underneath its sediment cover. Understanding such processes is important as hydrothermal fluids can withdraw significant amounts of heat from the oceanic lithosphere by lateral fluid flow through permeable basaltic crust having an age of up to 65 Ma. Basement outcrops in-between impermeable pelagic sediments permit recharge of oxic seawater and discharge of altered and slightly heated seawater. As the seawater migrates through the basaltic crust, it mines heat from the lithosphere, starts reacting with the basaltic rocks and feeds the microbial community with oxygen and nutrients. Single-channel seismic surveys, heat flow measurements, pore-water, sediment and manganese nodule sampling as well as video mapping were carried out for these investigations.