“How is it down there?” asks Dalton McGuinty.

“It’s dark,” comes the disembodied reply. The crowd titters. We’re assembled in a massive tent alongside the Niagara River — power-brokers in suits, construction workers in battered, decal-covered hard hats, untucked journalists in khakis — to witness the final “breakthrough.”

After a five-year battle with the fickle subterranean forces of the Niagara Escarpment, the long-awaited mega-tunnel under Niagara Falls is 99.99 percent complete. Just a metre and a half of solid rock remains, and the Ontario Premier is preparing to radio the “dig” signal to equipment foreman Kevin Collins, far underground in the control room of the world’s largest hard-rock tunnel boring machine.

A succession of speakers in the preceding hour had laid out the epic scale of the project: a $1.6 billion, 10.2-kilometre-long tunnel that has displaced enough rock to fill 100,000 dump trucks. It will ultimately funnel water from its intake above the falls to the Sir Adam Beck Generating Stations downstream at a rate that would fill an Olympicsized swimming pool every five seconds, spinning turbines to power 160,000 Ontario homes — a key plank in the province’s shift from fossil fuels to sustainable power. Urbanization long ago gobbled up the land alongside the Niagara River, making a canal impossible and the disruption of conventional drill-and-blast tunnelling a non-starter. So Ontario Power Generation (OPG) was forced to go deep. It commissioned the unprecedented boring machine, a wormlike behemoth that could chew through rock without waking the honeymooners slumbering above it, and, since tunnelling machines, like ships, are always female, dubbed her “Big Becky.”

Planning for the tunnel began in 1982. In the years that followed, routes were proposed, cores were drilled, rock samples were studied, impacts were assessed. Tunnelling finally started in 2006, but a trip underground is still a voyage into the unknown. “No matter how many investigations you do or how much money you spend up front, there’s always a certain uncertainty left,” says Ernst Gschnitzer, the project manager for Austrian construction giant Strabag, which is in charge of designing and building the tunnel. Sure enough, the carefully laid plans of an army of engineers began to unravel as soon as the tunnellers reached a troublesome layer called the Queenston shale, and as months passed and losses mounted, it began to look like yet another victory for geology in the age-old battle of man versus rock.

If you join the crowds strolling beside the Niagara River on a summer’s day and head upstream from Horseshoe Falls, you’ll pass a series of stately stone buildings fronted by majestic arches and columns: Canadian Niagara Power; Electrical Development Company; Ontario Power Company. All are now empty, vestiges of a long history that dates back to 1758, when Frenchman Daniel-Marie Chabert de Joncaire de Clausonne first harnessed the waters for his sawmill just above the American Falls.

Not surprisingly, the seemingly limitless torrent of the falls has played a central role in the history of hydro power and of electricity itself. One of the first hydroelectric generators in the world began powering 16 street lamps on the New York side of the river in 1881. A decade and a half later, when the Niagara Falls Power Company chose Nikola Tesla’s alternating-current (AC) transmission scheme for sending power to Buffalo instead of Thomas Edison’s direct-current (DC) scheme, the “War of the Currents” effectively ended and the template was established for how we transmit and use electricity to this day.

The current Sir Adam Beck Generating Stations are angular, modern structures jutting into the river about eight kilometres downstream from the falls, far from the cluster of old generating stations that proliferated above the falls at the start of the 20th century. That’s also where the Niagara Tunnel resurfaces, and it’s in a prefab office on the construction site there that I meet with Rick Everdell in August 2010 — nine months before the breakthrough ceremony — for my first tour of the then incomplete tunnel. Everdell, OPG’s project director for the tunnel, is a cheerful 59-year-old engineer who’s the last remaining member of the original team that started working on the project in 1982.

“We got smarter as time went on,” says Everdell, explaining the downstream location of the current power plant, which started operation in 1922. While the earlier plants harnessed the 55-metre drop over Horseshoe Falls, water for the first Beck plant was channelled through a canal that ran alongside the river but stayed on top of the Niagara Escarpment. When it finally reached the turbines 10 kilometres away, the water dropped 89 metres before rejoining the river — almost the entire height difference between Lake Erie and Lake Ontario, putting to use energy that was otherwise squandered in the rapids below the falls.

The next major step came in the 1950s, when the Sir Adam Beck II Generation Station added 16 more generators to the original 10. To feed the new station, the provincial utility dug two tunnels, starting above the falls using drill-and-blast construction, a disruptive approach that required five major construction shafts to be sunk along the route in the city of Niagara Falls. The tunnels ran only as far as St. David’s Buried Gorge, an ancient river valley that crosses the modern Niagara River a few kilometres below the Falls. Since the loose silt filling the gorge couldn’t be safely tunnelled through, the tunnels have to veer back up to the surface and the water completes its journey in a canal. The Americans, meanwhile, were also building tunnels to feed a new power plant across the river from the Beck stations, using the even more disruptive cut-and-cover technique, essentially digging a giant trench then covering it.

These days, as much as 4,000 cubic metres of water per second are diverted through various tunnels and canals around the falls to generate electricity. Canada and the United States signed a treaty in 1950 agreeing to ensure that at least 2,832 cubic metres per second flows over the falls between dawn and dusk during the tourist season from April to October. At other times, the minimum is cut in half, to 1,416 cubic metres per second. “The result is that the scenic flow requirement uses about one-third of the average Niagara River flow,” says Everdell. The rest is available for power generation, split 50-50 between the two countries.

The earliest hydroelectric systems in the area, which failed to capitalize on the full potential of the falls’ drop, were “repulsive to the engineer, because of the great waste,” wrote the renowned Scottish engineer George Forbes in 1895. In a similar vein, Everdell shows me a graph called a “flow-duration curve,” based on eight decades of river data. It reveals that Canada’s share of the water available for power exceeds the maximum capacity of the existing canal and tunnels about 65 percent of the time. “With the new tunnel in place, we’ll reduce that to about 15 percent of the time,” he says with evident satisfaction, “so there’s really not a whole lot of energy left in that water.”

The original plan, drawn up three decades ago, called for two new tunnels and a new underground generating station. Geotechnical studies started in 1988, and environmental approval for the full project was granted in 1998. But the current plan has been scaled back to only one tunnel and no additional generating capacity. “Essentially, what we’re doing is providing more fuel to the existing generating stations,” says Everdell. “That’s where you get the biggest bang for your buck.” The Ontario government finally gave the go-ahead in 2004; the billion-dollar design-build contract was awarded to Strabag in 2005; Big Becky was built from scratch in 12 months, and began tunnelling in September 2006. And then the trouble started.