Injection of vertical sheets of magma (dyke swarms) controls tectonic and volcanic processes on Earth and other planets. Yet we know little of the 3D structure of dyke swarms. We use seismic reflection data, which provides ultrasound-like images of Earth's subsurface, to study a dyke swarm in 3D for the first time. We show that (1) dyke injection occurred in the Late Jurassic, (2) our data support previous models of dyke shape, and (3) seismic data provides a new way to view and study dykes.