Pointing toward a much-needed future reservoir of freshwater for the island of Hawai'i in the face of climate-driven drought, electromagnetic images of the island have revealed multilayered basalt, ash and soil formations that serve as a previously unknown conduit to move freshwater offshore to the submarine flank of the island. According to the study by Eric Attias and colleagues, these freshwater-saturated layers may extend as much as 4 kilometers offshore. Attias et al. decided to look more deeply into the transport of freshwater through the island's complex geology after noting a 40% discrepancy in the predicted volume of fresh groundwater. The researchers looked for this missing freshwater using a technique called marine controlled-source electromagnetic (CSEM) imaging, which can distinguish seawater from freshwater through measurements of the electrical resistivity of rock formations. Their images mapped out a pattern of alternating ash/soil and basalt layers of differing porosity that trap layers of fresh groundwater while forcing out seawater. As these layers dip under the ocean, they form a 3.5 cubic kilometer cache of freshwater west of Hawai'i. Electromagnetic studies of other volcanic islands across the globe suggest the presence of similarly layered offshore hydrogeological formations, the researchers note, indicating potential new sources of freshwater for these islands as well.