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Description: When continents rift, magmatism can produce large volumes of melt that migrate upwards from deep below the Earth’s surface. To understand the role of magmatism in rifting, it is critical to understand how much melt is generated and how it transits the crust. Estimating melt volumes and pathways is, however, difficult, particularly in the lower crust where the resolution of geophysical techniques is limited. New broadband seismic reflection data allow us to image the 3-D geometry of magma crystallized in the lower crust (17.5-22 km depth) of the northern North Sea, in an area previously considered a magma-poor rift. The sub-horizontal magmatic intrusion is ~97 km long (N-S), ~62 km wide (E-W), and comprises several irregular lobes. The significant areal extent of the intrusion (~2700 km^2), as well as presence of intrusive steps indicate widespread lateral magma transport in the lower crust. We estimate that 472±161 km^3 of magma was emplaced within this intrusion, suggesting that the northern North Sea is more magmatic than previously thought.