While investigating limestone deposits in Gubbio, they discovered that some of the rocks were not aligned with the magnetic north pole, but in the opposite direction, implying the Earth undergoes geomagnetic reversal over time.

[1] Alvarez's hypothesis at the time conflicted over whether an impact was the cause or whether the iridium had been deposited by a supernova from a nearby star, which could have also killed most life on Earth due to gamma ray bursts and cosmic radiation.

In order to confirm or rule out this alternative hypothesis, Alvarez worked with Frank Asaro and Helen Michel to determine if the clay layer also contained plutonium-244, a distinctive isotope that a supernova would also have deposited if it had been the cause.

After investigating the effects of the 1883 eruption of Krakatoa, he determined that a large enough impact could force enough ash and dust into the atmosphere to block out the sun, leading to a global mass extinction.

[1] By 1980, evidence of the KT boundary and high iridium levels had been independently reported on at dozens of other sites, moving Alvarez's hypothesis toward a global hunt for the impact crater, in competition with several other scientists such as Jan Smit.

The discovery was made thanks to a graduate student named Alan Hildebrand that notified Alvarez of the evidence of a crater on the Yucatan Peninsula, which had never been published in the scientific literature by the Mexican petroleum geologists that had found it.

"[4] Clark R. Chapman, writing for Nature, stated that Alvarez's "slim" book can be read "in a single sitting and I recommend it highly – if only as a jumping-off point to other perspectives on this dramatic scientific revolution.

"[5] In a review for Scientific American of T. rex and the Crater of Doom and the opposing theories presented in The Great Dinosaur Extinction Controversy, Michael Benton advocated reading the book "for an excellent account of the pro-impact position and for insight into how scientists pose questions and seek to resolve them by sometimes roundabout means".

"[2] Los Angeles Times writer Dave A. Russell said that the book was "very well written and so engrossing that a reader with little or no background in the earth's geologic history will enjoy an easy and vastly entertaining summary of how we came to our present understanding of the past.