The asteroid impact that that saw off the dinosaurs.

Elegant, simple, generated by amateur(s), made experts look ridiculous.

Great answer, Anthony.

Best wishes, Paul.

A Nobel laureate is hardly an amateur. His son was a professional geologist, I believe.

For me Continental Drift which morphed into Plate Tectonics solved many more problems than the extinction of the dinosaurs. However, the asteroid impact did also sound the knell not only for the dinosaurs but also for Uniformitarianism, and helped introduce the new paradigm of Neocatastrophism.

I had thought about nominating continental drift, but I remember as a child looking at the globe and noticing how S America fitted into Africa, and I am sure I was not the only one, so it was not such an outrageous hypothesis. 

What I meant was that Luis A was not a geologist:

"In 1980 Alvarez and his son, geologist Walter Alvarez, along with nuclear chemists Frank Asaro and Helen Michel, "uncovered a calamity that literally shook the Earth and is one of the great discoveries about Earth's history".[1]

Walter Alvarez was doing geologic research in central Italy during the 1970s on the walls of a gorge whose limestone layers included strata both above and below the Cretaceous–Paleogene boundary (also called the K-Pg boundary), the boundary between those two geologic periods. Exactly at the boundary is a layer of clay. Walter told his 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.[1]

Alvarez had access to the nuclear chemists at the Lawrence Berkeley Laboratory ... 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.[1]

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."  

From Wikipedia

Luis A. may not have been a geologist, but he was a professional scientist (unlike me, who has not yet lost my amateur status.)

'Luis Walter Alvarez (June 13, 1911 – September 1, 1988) was an American experimental physicist, inventor, and professor who was awarded the Nobel Prize in Physics in 1968. The American Journal of Physics commented, "Luis Alvarez was one of the most brilliant and productive experimental physicists of the twentieth century."'

From Wikipedia

I have heard it argued that, since the idea was so outlandish [pun intended], (it ran counter to the prevailing paradigm of Uniformitarianism) that the Alvarez et al. (1980) paper would not have been published by Science, but for the fact that the lead author was a Nobel Prizewinner.