The nature and growth of magmatic plumbing systems is of fundamental importance to igneous geology. Traditionally magma chambers have been viewed as rapidly-emplaced bodies of molten rock or partially-crystallised “magma mush” connected to the surface by a narrow cylindrical conduit (referred to as the “balloon-and-straw” model). Recent data suggest, however, that magma chambers beneath volcanoes are formed incrementally through amalgamation of smaller intrusions. Here we present the first high-resolution 3D reconstruction of an ancient volcanic plumbing system as a large laccolithic complex. By integrating seismic reflection and gravity data we show that the ~200 km3 laccolith appears to have formed through partial amalgamation of smaller intrusions. The complex appears to have fed both surface volcanism and an extensive sill network beneath the volcanic edifice. Numerous sills are imaged within the volcanic conduit indicating that magma stalled at various levels during its ascent. Our results reveal for the first time the entire multi-component plumbing system within a large ancient shield volcano.