Geophysical anomalies clearly indicate a vast circular structure buried under approximately 1000 m of Cenozoic sediments of the Yucatan platform (SE Mexico). Cores drilled in the structure indicate the presence of suevite-like impact breccia, with abundant shocked minerals, and of a melt-breccia dated by 40 Ar/ 39 Ar at approximately 65 Ma. The lithology and age show that the Yucatan structure is thus the long sought Cretaceous-Tertiary (KT) impact crater. The suevite and melt-breccia are derived from the fracture and fusion of the lithologies present under the Yucatan platform at the time of impact, a succession of approximately 3 km of carbonate and evaporitic sediments overlying a Pan African age (550 Ma) silicate rich basement. The Chicxulub melts are chemically similar to the fragments of impact glasses found at the KT boundary all around the Gulf of Mexico. Impact glasses and shocked quartz form the base of a 2 and 4 metres thick coarse clastic sequence which marks the KT boundary from Alabama to Guatemala. These sands and silts were probably deposited, over a short period of time (a few days) by the gigantic tsunami waves triggered by the Chicxulub impact. Because of the target lithology, the Chicxulub event must have almost instantaneously released into the atmosphere huge quantities of water vapor, CO 2 and SO 2 . These components must have played a key role in the perturbation of the global Earth system and mass extinction taking place at the KT boundary.