Puerto Chicxulub is a fishing village on the north-west coast of the Yucatán Peninsula in eastern Mexico. The national petroleum company Pemex had been prospecting here, when in 1981 two of their geophysicists noticed magnetic and gravitational anomalies in the underlying rocks. The magnetic readings showed a high presence of iron, and the gravity was low, perhaps because of broken jumbles of rock: together these indicated an impact crater.
In 1991 this was identified as the long sought-after smoking gun in the 1980 Alvarez Theory of the K-T extinction, 65 million years ago, in which the dinosaurs and so many other creatures disappeared. (K means Cretaceous and T means Tertiary, the two geological periods separated by the event.)
The crater is enormous, over 200 km across, but has no visible trace, because it's over a kilometre down. It was identified because of rocks at the same age at the Brazos River in Texas, where a turbulent sandstone bed appears briefly between two larger layers of sediment, suggesting a tsunami had quickly deposited a great volume of material. This required an impact crater nearby, and the Chicxulub structure was identified as suitable. Tsunami deposits up to 4 m thick have since been found all around the Gulf of Mexico. Parts of the Caribbean are also covered by a layer of molten glass of exactly the same age.
It is estimated that 200 000 km3 of the earth's crust was instantly annihilated and ejected into the atmosphere. Worldwide, the K/T boundary shows up as a layer rich in iridium, a rare metal characteristic of asteroids. The Chicxulub event is the most powerful impact known since life began on Earth.
Cores drilled so as to show the impact layer reveal breccia, a shattered rock formation put back together under pressure, fractures in quartz, and an impact indicator called melt rock.
Further study of new cores is going on now. The new study will provide more precise information on the angle and intensity of the asteroid impact, and may decide whether it was strong enough to completely explain the extinction of the dinosaurs. The rival theory is that volcanic activity laying down the Deccan Traps in India released enough CO2 to change the global climate completely and cause the extinctions. It is possible that the impact caused the volcanism to begin.
http://dsaing.uqac.uquebec.ca/~mhiggins/MIAC/chicxulub.htm
http://earth.agu.org/revgeophys/claeys00/node8.html
http://www.agu.org/sci_soc/sharpton.html