Sit down, children, and let me tell you a story:  About 65.5 million years ago, whatever life forms there were present on this planet would have seen a terrible thing; they would have seen the end of the world.  A 10-15 km asteroid impacted the Earth in the Yucatan Peninsula with a force of over a billion times that of the nuclear blasts at Hiroshima and Nagasaki, combined.  The asteroid landed in a bed of gypsum, which would have released sulfur dioxide into the atmosphere.  There would have been a tremendous blast force, dust clouds, megatsunamis higher than any we’ve ever imagined, and an infrared pulse that would have lasted hours, killing by radiation.  Volcanic eruptions, global firestorms… well, I said it was the end of the world.  Anything specialized to an environment, that was picky about what it ate, or larger than a crocodile pretty much went extinct.  Anything that was small and could eat detritus (that would be non-living organic matter like fecal material and other organic trash) had a better chance.

Artist impression of Chicxulub Impact, NASA/JPL

Moving forward to the present time, in 1978 geophysicists working for the oil industry took a look at a strangely symmetrical crater at Chicxulub.  They read 1960s geological studies that theorized the crater was caused by an ancient impact.  The results of their exploration of the area were published in 1981, coincidentally the same year experimental physicist Luis Alvarez published his hypothesis that the K-T Extinction was caused by an impact.  Well, of course it was.  An international panel of 41 scientists have finally looked over all the evidence and have agreed that the extinction event was caused, at least in part, by the impact.  There were other global troubles at the time, which may or may not have caused some species to go extinct, but the event was definitely tipped over and put on the front burner by the impact.

Chixculub Crater, NASA/JPL

The Chicxulub crater itself is more than 180 km in diameter, which makes it one of the largest confirmed impact craters on Earth.  Material recovered from Chicxulub crater has been identified in part as shocked quartz, tektites, large deposits of iridium, andesite glass, and breccia.  You find these features in association with impacts.  I don’t think anyone really would argue that Chicxulub crater was caused by an impact.  Nothing else could have caused it.  It can’t be reproduced by natural Earth processes, and nothing causes shocked quartz except an impact.

Certainly, not every circular structure on the Earth is an impact crater (cough cough volcano), but this one definitely is from an impact; and when it formed, the world ended. I don’t know how many other times it’s happened here (definitely more than once), but I can tell you it will happen again.