Each time a volunteer editor adds a new fact to one of Wikipedia's over 44 million articles, they're required to cite where they learned it. The nonprofit Wikimedia Foundation, which oversees the encyclopedia, became interested in what kinds of sources editors rely on the most. A recent study conducted by the organization revealed something fascinating: A single academic paper, published by three Australian researchers in 2007, has been cited by Wikipedia editors over 2.8 million times—the next most popular work only shows up a little more than 21,000. And the researchers behind it didn't have a clue.

"Those numbers blew me away," says Brian Finlayson, one of the authors of the study and a retired geography professor at the University of Melbourne. "None of us had any idea about this. We didn't know Wikipedia collected this information or anything about it."

"It's a statistic that's hardly believable," says Thomas McMahon, a retired engineering professor at the same school and another co-author of the paper.

'None of us had any idea about this.' Brian Finlayson, University of Melbourne

It starts to make more sense, though, when you consider the focus of the research. Over a decade ago, Finalyson, McMahon, and Murray Peel created an updated map of world climate, based on the work of Russian-German climatologist Wladimir Köppen. In 1884, Köppen published one of the first maps of weather patterns around the world. It broke Earth into major climate classifications, like tropical rainforest, desert, and savanna. For over a century, Köppen's map informed the work of researchers and students from nearly every discipline. If you want to contrast, say, how animals behave in deserts versus highlands, you'd turn to Köppen's map. It was taught in schools across the globe, and became regarded as one of the most widely used academic resources.

In the 1950s, German climatologist Rudolf Geiger updated Köppen's climate map, creating what is sometimes referred to as the Köppen-Geiger climate classification system. Then, for decades, it went largely unmodernized. At least until the researchers from the University of Melbourne came along.

Around 2005, McMahon, Finlayson, and Peel (a PhD student at the time) were researching how streams flowed in different parts of the world. Their studies required learning about rainfall, and collecting data on general climate patterns across different regions, in order to make comparisons. Over time, the academics noticed they had amassed an enormous amount of data about climate across Earth—enough to re-draw the map Köppen had developed a century earlier. So they decided to create and publish an updated version.

"There's nothing scientifically new in it, we simply used Köppen's classification and added new data to it and then drew a world map," says Finlayson. "The reason it's so widely cited is because it's useful, and I think that's the important point about it, it's not that we suddenly dropped into the system this brand new thing that had never been done before."