NIAGARA FALLS, Ont. - Amid much fanfare, rubble and dust, the world's largest rock tunnel boring machine made a breakthrough deep below Niagara Falls, Ont., Friday and finally saw the light of day.

The massive, 4,000-tonne machine, called Big Becky, had been boring a 10.2-kilometre long, 14.4-metre high tunnel under the city since 2006.

Crowds of workers, politicians, executives and students cheered when, after 45 minutes of drilling, falling rocks gave way to the giant boring machine.

"At the outset the challenge was, in a word, intimidating," said Premier Dalton McGuinty, who was on hand for the event.

"Lots of people from lots of places have gone over the Falls in barrels, only Ontarians have gone under them."

McGuinty launched the breakthrough after giving the machine's driver the go-ahead to start Big Becky for the final cut and breakthrough.

"It's a great feeling to hear the machine from this side and watch it come through," said Frank Caroli, who worked on the project.

Being part of such a massive undertaking was at times challenging and involved long hours, Caroli said, as shifts inside the tunnel would often last 11 hours and go for 11 consecutive days.

"When you come out and you see the sun and you feel those rays, what a feeling that is," Caroli said. "You really didn't know how much you miss the sun until you don't have it anymore."

Big Becky was named by then-six-grader Rachel Wiens, who brainstormed names along with her classmates in St. Catharines. Her suggestion, Big Beck, was eventually changed to Big Becky, but remained a nod to Sir Adam Beck, who was instrumental in establishing public power in Ontario.

"It's pretty cool," Wiens, now 17, said of the naming rights. "It's an interesting experience."

The tunnel is part of a hydroelectric infrastructure project that will be completed in 2013. It has been churning along about 90 to 140 metres below the city since 2006.

The tunnel -- about one and a half times as large in diameter as the Chunnel between England and France -- will be used to redirect water from the Niagara River to hydro generating stations, creating enough electricity to power 160,000 homes.

Critics have complained the project is four years late and half a billion dollars over budget, as construction crews hit harder than expected rock early on, which slowed things down and added to cost overruns.

"This is just another example of the McGuinty government failing to watch the bottom line," said Progressive Conservative Leader Tim Hudak.

"This project is now four years late and $600 million over budget. That's going to hit hydro bills hard."

McGuinty, however, insisted the $1.6-billion project was worth it.

"This is clean hydroelectric power, it's a project that's going endure for a hundred years, so I think it was well worth the investment."