There are now many lines of evidence to prove that a relatively large impact happened 65 My ago.
The iridium excess in the 65 My-old soil layer has been confirmed at many points around the world.The same soil layer contains grains of quartz that were deformed by high shock pressures, as would occur in a giant explosion. (The deformation is a microscopic structure called "twinning," in the crystals).The same soil layer contains enough soot to correspond to burnding down all of the forests of the world. This suggests that massive fires were touched off at the time of impact.The same soil layer, especially around the Gulf of Mexico, contains massive deposits of tumbled boulders, as would be generated in a large tsunami, or "tidal wave." The geographic distribution of tsunami deposits suggest the impact was in the Caribbean area.After a decade of searching, scientists in 1990 identified the crater associated with this material. It is no longer visible on the surface of the Earth, but is buried under sediments. It straddles the coast of Yucatan. It is revealed by mapping the strength of the gravity field over that area, and by drilling; it has been dated to 65 My old.Astronomers have charted numerous asteroids that cross Earth's orbit. From studies of orbit statistics, it is estimated that asteroids of 10 km size can hit the earth roughly every 100 My or so -- which fits with the idea that we actually did get hit 65 My ago by an object this size. (Smaller hits are much more common). How Was the Impact Event Discovered?
In the late 1970s, a team of geochemists headed by Luis Alvarez and his son Walter Alvarez, both with connections to the University of California at Berkeley, was studying chemical changes in soil layers corresponding to breaks in the fossil record. In the soil layer that separates the Mesozoic Era from the Cenozoic Era, dating 65 My ago, they found an excess of the element iridium, which is common in meteorites. Meteorites are believed to be fragments of asteroids. Therefore, the Alvarez team theorized that an asteroid had hit Earth at this time, and that the debris ejected from the explosion were spread in the soil layer.
For about ten years, this theory was extremely controversial. However, compelling evidence has accumulated to support the theory. From https://www.psi.edu
