It’s been known for a long time that the dinosaurs and many other species went extinct fairly quickly. It was likely due to a fairly widespread and catastrophic event, but what?
Luis Alvarez was a physicist on the Manhattan Project, his son Walter a geologist. When Walter showed his father some sedimentary rock of an age associated with that extinction Luis thought that an examination of radioactive isotopes might help reveal what happened. Their investigation showed unusually high levels of tritium compared to both older and newer layers. They knew that such high levels didn’t occur naturally on Earth but were consistent with would be expected if a very large asteroid were to strike.
The proof of this hypothesis hinged on being able to determine its size and where it actually hit. That discovery would eventually come but unfortunately Luis had passed away.