Construction began in 1957, and Furnas was the first large dam Brazil ever built — the project that taught the country's power sector what a project of this scale actually involves. The contractor was Wimpey Construction, a British firm, and the work went up in the canyon of the Grande River upstream of its junction with the Sapucaí. The finished structure stands 127 metres tall and 550 metres long, with a crest just fifteen metres wide: a thin blade of concrete and earth holding back something the size of a small inland sea. A settlement grew beside the site to house the workforce and simply took the dam's name, as company towns do. Completion came in 1963, six years after the first excavation. The Grande is one of the two great headwater rivers of the Paraná, so what Brazil learned at Furnas it went on to apply the length of that basin, dam after dam, for the next thirty years.

Filling a reservoir on paper and filling one in the field are different exercises. The water reached the lower streets of Guapé in January 1963 and kept coming. The company rebuilt the town on higher ground with new infrastructure, and did the same for the village of Barra Velha in what is now São José da Barra, but the schedule outran the paperwork. Families who had been told they had years to arrange a move found the lake at their doorsteps within months, and some of them had to be pushed out ahead of it. Nobody in Minas talks about Furnas for very long without arriving at this — the farms, the churchyards, the river towns that are still down there under the boat traffic.

Eight Francis turbines of 152 megawatts each give Furnas an installed capacity of 1,216 megawatts, which was a very large number in 1963 and remains a serious one now. But generation is only half the job. The reservoir's other purpose is regulation: holding back the summer floods of the Grande and releasing water at a controlled rate through the dry months, which lets the whole cascade of power stations downstream run on a predictable river rather than a seasonal one. Normal operating level sits around 768 metres above sea level, and the stored volume — 22,590 million cubic metres — is the buffer that makes that possible. Furnas does not just make electricity. It makes the river behave.

Half a century on, the lake's economy is as much about weekends as watts. The drowned gorges near Capitólio turned out to be spectacular — sheer sandstone corridors barely wide enough for two boats, with waterfalls dropping straight into deep green water — and speedboat tours, diving and lakeside pousadas grew up around them. The shoreline is young, though, in the way that only an artificial shoreline can be: cliffs that spent millions of years weathering above a river now weather above a boat lane. On 8 January 2022 one of those walls collapsed onto four tourist boats and killed ten people. The canyons reopened under strict new limits on boat numbers, speed and time. The Sea of Minas is a working landscape twice over — once for the grid, once for the visitors — and neither role was in the original drawings.