Fig. 3

a Location of Katla in south Iceland (red box) and the volcanic zones distributed along the divergent plate boundaries (dark grey zone). The glaciers are coloured in cyan and the closed lines depict the volcanic centres. b The Mýrdalsjökull glacier (cyan) and the subglacial Katla volcano. The glacier contours are spaced every 100 m. The borders of the caldera are indicated by a purple hatched line. The watersheds beneath the icecap are indicated by the grey dashed lines and the rivers are in dark blue. The purple star shows the exercised centre of the seismic activity at Gvendarfell. The red circle within the caldera indicates the Mogi source of deformation associated with the exercise’s main eruption and the red star corresponds to the eruption site. The geothermal cauldrons of relevance to the exercise are shown by orange circles. The ring road is plotted in light brown and the green triangle indicates a road bridge that was swept away on day 1 of the exercise