Most of the magma erupted at mid-ocean ridges is stored in a mid-crustal melt lens that lies at the boundary between sheeted dikes and gabbros. Nevertheless, images of the magma pathways linking this melt lens to the overlying eruption site have remained elusive. Here, we have used seismic methods to image the thickest magma reservoir observed beneath any spreading center to date, which is principally attributed to the juxtaposition of the Juan de Fuca Ridge with the Cobb hotspot (northwestern USA). Our results reveal a complex melt body, which is ∼14 km long, 3 km wide, and up to 1 km thick, beneath the summit caldera. The estimated volume of the reservoir is 18–30 km3, more than two orders of magnitude greater than the erupted magma volumes of either the A.D. 1998 or 2011 eruption. Our images show a network of sub-horizontal to shallow-dipping (<30°) features that we interpret as pathways facilitating melt transport from the magma reservoir to the eruption sites.