A one-centimeter layer of Italian clay carrying thirty times too much iridium, the sliver of evidence that turned the dinosaur extinction from a slow fade into a murder investigation.
Documented
The Gubbio spike
In 1980, Alvarez, Alvarez, Asaro and Michel reported in Science that the thin clay marking the Cretaceous-Tertiary boundary held iridium at 30 times background in Italy, 160 times in Denmark, and 20 times in New Zealand. They proposed a 10 ± 4 kilometer asteroid whose dust would shut down global photosynthesis for years. They had a body, a chemical fingerprint, and no crater.
Assessment: Documented: the 1980 Science paper and its three-site iridium data. The extinction became a forensics problem that day.
Stories
The weapon found later
Popular retellings compress a decade of detective work into an instant discovery. In reality the 180-kilometer Chicxulub structure had been mapped by Pemex oil geophysicists since the 1970s, and was matched to the extinction only in 1991, when Hildebrand and colleagues tied its buried ring to the boundary layer.
Assessment: Stories: the crater was under exploration before anyone knew it was a crater. The solution waited in a petroleum archive.
Interesting claims
An accident of preservation
The anomaly survived because deep-sea limestone quietly records fallout; most of Earth's surface erases such layers within geological moments. The largest event in the history of animal life was nearly invisible, and was found only because one Italian hillside kept the receipt.
Assessment: Interesting: the evidence chain for the worst day in Earth's history hangs on one continent's worth of unpaved road cuts.
Sources and further reading
Alvarez, Alvarez, Asaro and Michel, Science 208:1095-1108 (1980), 'Extraterrestrial Cause for the Cretaceous-Tertiary Extinction'
Hildebrand et al., Geology 19:867-871 (1991), matching the Chicxulub crater to the boundary
Pemex gravity surveys and Glen Penfield's 1970s ring data, as documented in later reviews
Worth asking
If one centimeter of clay can rewrite sixty-five million years, what else is sitting in a road cut, unscraped?