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Globally, instrumentally based assessments of tsunamigenic potential of subduction zones have underestimated the magnitude and frequency of great events because of their short time record. Historical and sediment records of large earthquakes and tsunamis have expanded the temporal data and estimated size of these events. instrumental records suggests that the Mexican Subduction earthquakes produce relatively small tsunamis, however historical records and now geologic evidence suggest that great earthquakes and tsunamis have whipped the Pacific coast of Mexico in the past. the sediment marks of centuries old-tsunamis validate historical records and indicate that large tsunamigenic earthquakes have shaken the Guerrero-oaxaca region in southern Mexico and had an impact on a bigger stretch of the coast than previously suspected. We present the first geologic evidence of great tsunamis near the trench of a subduction zone previously underestimated as potential source for great earthquakes and tsunamis. Two sandy tsunami deposits extend over 1.5 km inland of the coast. The youngest tsunami deposit is associated with the 1787 great earthquake, M 8.6, producing a giant tsunami that poured over the coast flooding 500 km alongshore the Mexican Pacific coast and up to 6 km inland. The oldest event from a less historically documented event occurred in 1537. The 1787 earthquake, and tsunami and a probable predecessor in 1537, suggest a plausible recurrence interval of 250 years. We prove that the common believe that great tsunamis do not occur on the Mexican Pacific coast cannot be sustained. Worldwide, instrumentally based assessments of tsunamigenic potential of subduction zones have underestimated the magnitude and frequency of great events 1-3 , to some extent because great earthquakes and tsunamis are infrequent 1-5 and instrumental seismic data are relatively short. Historical and sediment records of large earthquakes and tsunamis have expanded the temporal data and estimated size of these events but mainly where direct observations of great tsunamis have been possible 1,3,4,6. This means that little is still known of great earthquakes and tsunami generation potential of other subduction zones 7-11. Additionally, at the centre of the problem is still the question as to whether subduction zones, despite their relatively short instrumentally seismic history, could generate great earthquakes and tsunamis. We reveal the first geologic evidence, and validate historical records, of great tsunamis and earthquakes near the trench of the Mexican subduction zone previously underestimated as potential source for great earthquakes and tsunamis. Here, we focus on the Corralero coastal plain, in southwestern Mexico, where a great earthquake, M 8.6, triggered a giant tsunami that poured over the coast of Oaxaca, Guerrero, and Chiapas, flooding 500 km along open