The lake might never have happened without Robert Menzies. Australia's longest-serving prime minister had once regarded the unfinished capital as a national embarrassment, and over time his contempt curdled into a kind of crusade. He sacked two ministers he felt lacked intensity for the job. He created a powerful new development commission, blocked schemes that strayed from Griffin's intent, and denounced the "moaning" of the project's opponents. When the lake was at last inaugurated on 17 October 1964 amid fireworks and a flotilla of sailing boats, the obvious gesture would have been to name it for Menzies himself - and it was proposed. He refused. He insisted it carry the name of Griffin, the architect who had imagined it and never lived to see it fill.

Beneath its placid surface, the lake is a feat of engineering. It stretches 11 kilometres long and as much as 1.2 kilometres wide, holding some 33 million cubic metres of water against the 33-metre wall of Scrivener Dam - named for Charles Scrivener, the surveyor who chose Canberra's site in 1909 partly because the land could be flooded "for ornamental purposes at reasonable cost." The dam is built to withstand a flood of the kind expected once in five thousand years. Crews dug the entire lakebed at least two metres deep, both to clear boat keels and to discourage mosquitoes and weeds. In times of drought, water can be released from the Googong Dam upstream to keep the level steady. It is, in the end, an artificial body of water pretending convincingly to be a natural one.

Griffin wanted the lake to be the foreground of the capital, and so it became. Along its edges rose the institutions of the country: the National Library opened in 1968, the High Court and the National Gallery followed around 1980, and in 1988 the new Parliament House crowned Capital Hill behind them. On Queen Elizabeth II Island stands the National Carillon, originally fifty-three bronze bells given by Britain for the city's fiftieth anniversary and since expanded to fifty-seven, their music drifting across the water. The Captain Cook Memorial Jet can throw a column of spray more than a hundred metres into the air. Black swans gather on the East Basin in flotillas, and the murky water hides Murray cod and golden perch stocked to revive the river's depleted runs.

To Canberrans the lake is less a monument than a backyard. A shared path runs almost the full 40-kilometre shoreline, broken into three loops by the two great bridges, so a runner or cyclist can take on the 3.7-kilometre "bridge to bridge" or the full circuit of roughly 32 kilometres. Rowers slice across the calm of early morning; sailors, paddleboarders and dragon-boat crews share the water on weekends. Each spring, Floriade - the largest flower festival in the country - blankets the lakeside Commonwealth Park in a million blooms. Swimming has grown rarer, discouraged by cold water and the blue-green algae that bloom in warm spells. Still, on a clear Canberra evening, the whole improbable, long-delayed vision finally makes sense.