Twice a day, the Bay of Fundy, the long inlet separating Nova Scotia from New Brunswick, fills and empties with a force unmatched anywhere on Earth. The bay owes this distinction to an accident of geometry: its funnelling shape and its length happen to align with the natural rhythm at which its waters swing back and forth, so that each incoming tide reinforces the last, much as a child pumping a swing adds to its arc. The result is a tidal range that can exceed sixteen metres between low and high water, moving a volume of seawater on every cycle that is said to rival the combined flow of all the world's rivers. For centuries this daily surge was regarded as little more than a hazard to shipping and a spectacle for visitors.

The notion of turning that surge into electricity is far from new. As early as the colonial period, settlers along the bay built small tidal mills, trapping the rising water behind a dam and releasing it through a wheel as it fell. The modern descendant of that idea, a barrage generating station opened at Annapolis Royal in 1984, worked on precisely the same principle at a far larger scale, driving turbines with water held back by a wall across a river mouth. Yet the barrage approach carried a heavy penalty: sealing off an estuary drowned mudflats, blocked the passage of fish, and altered the very tides it exploited, and no second station of the kind was ever built in the region.