A new onshore-offshore three-dimensional seismic velocity model for the Island of Hawaii reveals a massive buried rift zone within Mauna Loa's southeast flank, introduced here as the Ninole rift zone. This feature extends more than 60 km south of Mauna Loa's summit, spans a depth range of ∼2–14 km below sea level, and is the probable source of the 100–200 ka Ninole volcanics in several prominent erosional hills. The ancient rift zone may stabilize Mauna Loa's southeast flank, focusing recent volcanic activity and deformation onto the unbuttressed west flank. The upper portion of the Ninole rift zone appears to have migrated westward over time, possibly triggered by landsliding, causing its eventual abandonment in preference to Mauna Loa's present-day southwest rift zone. Subsequently, the lower southwest rift zone broke away, tracking rift intrusions along the trace of the Kahuku detachment fault. Similar rift zone migration is thought to be under way at Kilauea volcano, and may one day lead to the abandonment of the east rift zone. Such rift zone reconfiguration is a reflection of changing stress conditions within growing volcanoes; it is probably much more common than previously assumed, and may enable the growth of large volcanic edifices such as Mauna Loa.