On 9 July A.D. 551, a large earthquake, followed by a tsunami, destroyed most of the coastal cities of Phoenicia (modern-day Lebanon). Tripoli is reported to have “drowned,” and Berytus (Beirut) did not recover for nearly 1300 yr afterwards. Geophysical data from the Shalimar survey unveil the source of this event, which may have had a moment magnitude (Mw) of 7.5 and was arguably one of the most devastating historical submarine earthquakes in the eastern Mediterranean: rupture of the offshore, hitherto unknown, ∼100–150-km-long active, east-dipping Mount Lebanon thrust. Deep-towed sonar swaths along the base of prominent bathymetric escarpments reveal fresh, west-facing seismic scarps that cut the sediment-smoothed seafloor. The Mount Lebanon thrust trace comes closest (∼8 km) to the coast between Beirut and Enfeh, where, as 13 14C-calibrated ages indicate, a shoreline-fringing vermetid bench suddenly emerged by ∼80 cm in the sixth century A.D. At Tabarja, the regular vertical separation (∼1 m) of higher fossil benches suggests uplift by three more earthquakes of comparable size since the Holocene sea level reached a maximum ca. 7–6 ka, implying a 1500-1750 yr recurrence time. Unabated thrusting on the Mount Lebanon thrust likely drove the growth of Mount Lebanon since the late Miocene.