It’s hard to overstate the massive proportions of Belo Monte. When completed, it will be the world’s third-largest dam. Set in the heart of the Brazilian Amazon, it will funnel water through 18 turbines, each with the diameter of the Space Shuttle. When the last one is switched on in 2019, Belo Monte will have a capacity of 11,233 megawatts—enough to supply power to 40 percent of Brazil’s households. Construction crews, which began work in 2011, will have to dig up almost twice as much dirt as was removed to build China’s Three Gorges Dam, and truck in enough steel to build 16 Eiffel Towers, according to the dam’s developers. About 26,000 laborers toil in shifts around the clock.

Massive is a word that also applies to the hydropower project’s price. Norte Energia, the consortium awarded a 35-year concession to operate the dam, pegs the cost at 29 billion reais ($12.3 billion), up from 25.8 billion reais when it bid for the contract in 2010. Eletrobras, the state-owned utility holding company that is the lead partner in the consortium, estimated the cost at just 8 billion reais in 2008.

Photograph by Taylor Weidman/zReportage via ZUMA PressPhoto Essay – Last of Eden: In the Shadow of Brazil’s Largest Construction Site

Brazilian President Dilma Rousseff, who was energy minister from 2003 to 2005, has publicly defended Belo Monte against attacks from environmentalists and indigenous groups, calling it a “social investment” that will improve the lives of those in the Amazon. Yet a study published this month suggests that the government-run companies footing most of the bill for Belo Monte may never recoup their investment.

Bent Flyvbjerg, an economic geographer on the faculty at Oxford University, and Atif Ansar, a lecturer at Oxford, surveyed 245 dam projects across 65 countries and found they had average cost and schedule overruns of 96 percent and 44 percent, respectively. “We estimate that half of the 245 large dams we’ve studied are nonviable, meaning they won’t be able to redeem the cost of building them,” Flyvbjerg said in an interview. “Belo Monte is one of the striking examples, and it’s not even finished yet,” says the academic, who estimates the complex could wind up costing $27 billion. Brazil’s Itaipu Dam, built in the 1970s, which had cost overruns of 240 percent, impaired the nation’s finances for three decades, according to Flyvbjerg.

Private companies have been reluctant to invest in Belo Monte, because regulators have capped electricity tariffs at a level that will make the project uneconomical, says Celio Bermann, a professor at the University of São Paulo’s institute of energy and environment. Builders Odebrecht and Camargo Corrêa declined to bid on the operating concession, saying the necessary “economic and financial conditions” were lacking. Instead, they joined the consortium of engineering and construction companies that won the 14 billion-reais contract to build Belo Monte—ensuring that they’ll get paid so long as the complex gets built.

That leaves the stakeholders in Norte Energia—mostly state-run firms and their pension funds—as the guarantor of a 22.5 billion-reais loan from Brazil’s development bank, BNDES. João Pimentel, the consortium’s social and environmental affairs director, dismisses any suggestion that the partners are destined to lose money. “The investors analyzed the project and decided the return is good,” he says.