English: "a, Map of sampled proglacial groundwater springs (green circles) and springs identified by satellite images but not sampled (pink circles). The outlined area designates the full study region (6,268 km2). (The retreat of glacial termini since 1936 can be viewed in ref. 14 to contextualize the magnitude of glacial retreat and the increasing size of the sampled forefields.) b, Methane concentrations (nM) of sampled proglacial groundwater springs displayed by bubble size over a geological map of central Svalbard. Black lines on the map designate the locations of geological faults. The red line indicates the location of the A–A’ cross section depicted in c. c, Cross section displaying the geological layers of the study region. Basemaps in a,b adapted with permission under a Creative Commons license CC BY 4.0. Panel c adapted with permission ref. 49 under a Creative Commons license CC BY."