In 1755, the great Lisbon earthquake (estimated magnitude 8.7) killed 60,000 people in southern Iberia and northwest Morocco and generated a 5- to 10-m-high tsunami wave. In his Perspective, Gutscher discusses the current research on the tectonic source of the earthquake. Recent evidence suggests that an eastward-dipping subduction zone beneath Gibraltar may have caused the earthquake. Though no interplate earthquakes have been recorded here, evidence from mud volcanoes, seismic images of deformed sediments, and heat flow data suggest that subduction is active and that the seismogenic fault zone may be locked. Numerous cruises planned in 2004 and 2005 may help to provide a definitive answer.