The M(w) = 8.3 deep (636 kilometers) Bolivian earthquake of 9 June 1994 was the largest deep-focus earthquake ever recorded. Seismic data from permanent stations plus portable instruments in South America show that rupture occurred on a horizontal plane and extended at least 30 by 50 kilometers. Rupture proceeded at 1 to 3 kilometers per second along the down-dip azimuth of the slab and penetrated through more than a third of the slab thickness. This extent is more than three times that expected for a metastable wedge of olivine at the core of the slab, and thus appears to be incompatible with an origin by transformational faulting. These large events may instead represent slip on preserved zones of weakness established in oceanic lithosphere at the Earth’s surface.