father that the layer marked where the dinosaurs and much else became extinct and that nobody knew why, or what the clay was about—it was a big mystery and he intended, to solve it. Alvarez had access to the nuclear chemists at the Lawrence Berkeley Laboratory and was able to work with Frank Asaro and Helen Michel, who used the technique of neutron activation analysis. In 1980, Alvarez, Alvarez, Asaro, and Michel published a seminal paper proposing an extraterrestrial cause for the Cretaceous-Paleogene extinction (then called the Cretaceous-Tertiary extinction). In the years following the publication of their article, the clay was also found to contain soot, glassy spherules, shocked quartz crystals, microscopic diamonds, and rare minerals formed only under conditions of great temperature and pressure. Publication of the 1980 paper brought criticism from the geologic community, and an often acrimonious scientific debate ensued. Ten years later, and after Alvarez's death, evidence of a large impact crater called Chicxulub was found off the coast of Mexico, providing support for the theory. Other researchers later found that the end-Cretaceous extinction of the dinosaurs may have occurred rapidly in geologic terms, over thousands of years, rather than millions of years as had previously been supposed. Others continue to study alternative extinction causes such as increased volcanism, particularly the massive Deccan Traps eruptions that occurred around the same time, and climate change, checking against the fossil record. However, on March 4, 2010, a panel of 41 scientists agreed that the Chicxulub asteroid impact triggered the mass extinction. Aviation In his autobiography, Alvarez said, "I think of myself as having had two separate careers, one in science and one in aviation. I've found the two almost equally rewarding." An important contributor to this was his enjoyment of flying. He learned to fly in 1933, later earning instrument and multi-engine ratings. Over the next 50 years he accumulated over 1000 hours of flight time, most of it as pilot in command. He said, "I found few activities as satisfying as being pilot in command with responsibility for my passengers' lives." Alvarez made numerous professional contributions to aviation. During World War II he led the development of multiple aviation-related technologies. Several of his projects are described above, including Ground Controlled Approach (GCA) for which he was awarded the Collier Trophy in 1945. He also held the basic patent for the radar transponder, for which he assigned rights