Harrat Lunayyir is one of the most volcanically active recent basaltic fields in western Saudi Arabia. A period of substantial seismic unrest, featuring more than 30,000 local events, occurred in Harrat Lunayyir in April–June 2009. Although this crisis was presumably related to ongoing magma activity, it ended without any surface volcanic activity. We create new tomographic models of P and S velocities (VP and VS) and use them to explain the causes of this unrest and the reasons the eruption failed. A large seismic anomaly of high VP/VS ratio below 7 km depth coincides with the locations of more than 50 recent cinder cones with ages of older than 100 ka, and is interpreted as a steady-state magma reservoir. We also identify another seismic anomaly at depths below 15 km, which is interpreted as a conduit for fluids and melts from deeper sources. Because the location of this conduit is slightly outside the main reservoir, some of the incoming material was dispersed. As a result, the activation of the crustal reservoir was not sufficiently strong to pierce the rigid basaltic cover and cause an actual eruption during the crisis in 2009.