The Birkeland-Eyde process worked by mimicking lightning. An electric arc heated air to extreme temperatures, forcing nitrogen and oxygen to combine into nitric oxide, which could then be processed into calcium nitrate fertilizer. The idea was tested first at the Royal Frederick University in Kristiania, then moved to progressively larger facilities as its power demands outgrew each location -- a warehouse in the capital, a testing station in Maridalen powered by the Hammeren hydroelectric plant, and finally Vassmoen near Arendal, where the first industrial-scale arc furnaces were built. On November 18, 1905, the process was presented at the Technische Universitaet Berlin. Birkeland and Eyde had proven that Scandinavian waterfalls could replace South American mining as the world's source of agricultural nitrogen.

To commercialize their invention, Birkeland and Eyde founded Norsk Hydro -- the Norwegian Hydroelectric Nitrogen Share-Company -- with financing from Sweden's influential Wallenberg family and the French bank Banque de Paris et des Pays-Bas. The first production facility was established at Notodden, powered by the Tinfos I hydroelectric station leased from the company Tinfos AS. As production scaled up, the operation expanded into the Vestfjorddalen valley, where the Vemork power station was built. Railways were constructed to connect the factories: the Rjukan Line and the Tinnoset Line, with train ferries crossing Lake Tinnsjo. Around the plants, entire towns took shape -- worker housing, schools, social institutions, all built by the companies that needed the labor.

The Vemork hydroelectric plant, designed by architect Olaf Nordhagen and completed in 1911, was the largest in the world at the time. Its massive turbines powered not only fertilizer production but also a hydrogen plant that became the first facility to mass-produce heavy water -- a substance that would acquire terrifying strategic importance during World War II. Heavy water was critical for certain approaches to nuclear fission research, and the Nazi occupation of Norway gave Germany access to Vemork's output. In February 1943, Norwegian commandos carried out Operation Gunnerside, sabotaging the heavy water cells in one of the war's most consequential raids. Today, the Vemork building houses the Norwegian Industrial Workers Museum, its turbine hall still intact.

What makes the Rjukan-Notodden site exceptional as a World Heritage Site is not any single building but the completeness of the industrial system it preserves. Hydroelectric dams, power stations, transmission lines, factories, railways, ferries, and the towns that grew to support them -- all remain legible in the landscape. The heritage railway still runs. The factory towns still house families. Lake Heddalsvatnet still reflects the infrastructure along its shores. The site was placed on UNESCO's tentative list in June 2009, alongside the similar Odda-Tyssedal complex, and received full inscription on July 5, 2015. It stands as evidence of how Norway's geography -- steep valleys, abundant water, sparse population -- became the raw material for an industrial revolution that fed a continent.