The Parana River is the seventh-largest in the world, and for decades both Brazil and Paraguay eyed its hydroelectric potential with competing ambitions. The diplomatic breakthrough came on July 22, 1966, when the two nations signed the Iguacu Act, declaring mutual interest in harnessing the river's power. Seven years of negotiation followed before the Itaipu Treaty was signed on April 26, 1973, creating the Itaipu Binacional entity to manage the project. Argentina, downstream and wary of being left dry, contested the dam until all three nations signed the Acordo Tripartite in 1979, establishing how much river levels could fluctuate. The agreement turned a potential flashpoint into the foundation of South American regional cooperation -- though it took threats of conflict to get there.

Construction began in 1975, and forty thousand workers descended on the border region to attempt something that had never been done at this scale. On October 14, 1978, engineers rerouted the entire Parana River, diverting the seventh-largest river on Earth so they could build on its exposed riverbed. Fifty million tonnes of earth and rock were moved. The concrete poured into Itaipu's four interconnected dams -- an earth-fill section, a rock-fill section, a concrete buttress dam, and a concrete wing dam -- would have been enough to build 210 Maracana stadiums. The total excavation volume was 8.5 times greater than the Channel Tunnel's. By the time the first generating unit spun to life on May 5, 1984, nearly a decade of labor had produced one of the most expensive objects ever built.

When the reservoir began filling on October 13, 1982, the rising water swallowed the Guaira Falls -- 18 cataracts clustered in seven groups, whose roar could be heard 20 miles away. Months before, an overcrowded footbridge overlooking the falls collapsed as tourists rushed for a final glimpse, killing 80 people. The Brazilian government dissolved the national park that had protected the falls. Beyond the human displacement and the lost landscape, the dam destroyed a natural barrier that had separated freshwater ecosystems for millennia. More than 30 fish species formerly confined below the falls invaded the upper basin, disrupting an ecosystem whose endemic species had evolved in isolation. The dam's director offered cold comfort, claiming the dam's spillway would be "a substitute for the beauty" of the falls.

Itaipu's 20 generating units are an engineering oddity: ten run at 50 hertz for Paraguay, ten at 60 hertz for Brazil. Paraguay's electrical demand is a fraction of the dam's output, so most Paraguayan-side generation flows east through two 600-kilovolt high-voltage DC transmission lines, each stretching 800 kilometers to the Sao Paulo and Rio de Janeiro region. In 2009, the fragility of this arrangement became apparent when a storm severed transmission lines, blacking out the entire country of Paraguay for 15 minutes and plunging 50 million Brazilians into darkness for over two hours. The dam itself was undamaged -- but the infrastructure connecting it to civilization failed in minutes, a reminder that generating power and delivering it are different problems entirely.

For Paraguay, Itaipu has been both a lifeline and a grievance. The 1973 treaty obligated Paraguay to sell its surplus electricity exclusively to Brazil at below-market rates -- a provision that many Paraguayans viewed as economic exploitation by a much larger neighbor. President Fernando Lugo made renegotiation a cornerstone of his 2008 campaign. In 2009, Brazil agreed to fairer pricing and allowed Paraguay to sell excess power directly to Brazilian companies rather than through the state monopoly. The treaty expired in 2023, opening a new chapter in the binational relationship. Meanwhile, Itaipu continues to produce staggering amounts of energy -- 103.1 terawatt-hours in 2016, a world record at the time. Only China's Three Gorges Dam, with 60 percent more installed capacity, has since surpassed it. In the sounding stone's shadow, the arguments over who benefits continue.