A whitewater wonder visited by everyone from Marilyn Monroe to Mark Twain, Niagara Falls has been a magnet drawing global travelers for at least two centuries. But until this year, a huge tunnel buried deep below the cascade has been off-limits to visitors.

The rocks beneath the gigantic triple waterfall that straddles the border between the US state of New York and Canada's province of Ontario are honeycombed with chambers carved out to harness the powerful forces of nature thundering overhead.

Since July 2022, it's been part of tours of the decommissioned Niagara Parks Power Station tour which began a year earlier. Exploring it offers a fascinating glimpse into pioneering work that helped bring this corner of North America into the modern age.

The power station, which operated from 1905 until 2006, diverted water from the mighty Niagara River to run giant generators that electrified regional industry and contributed to the nearby Great Lakes port of Buffalo becoming known as the City of Light.

The region around the waterfall, according to station tour guide Elena Zoric, was once a hub of activity for businessmen who wanted to cash in on harnessing hydro power.

The Adams hydroelectric power plant was the first to open, operating on the US side from 1895 to 1961. On the Canadian side, the Ontario Power Company operated from 1905 to 1999, and the Toronto Power Generating Station from 1906 to 1974.

Today, the Niagara Parks station is the world's only fully intact hydroelectric plant of its era. Originally operated by the Canadian Niagara Power Company, it used Westinghouse generators to create alternating currents patented by inventor Nikola Tesla -- cutting-edge technology at that time.

Before reaching the tunnel, visitors to the power station are shown a scale model of the massive engineering works that went into converting the pounding waters into electricity.

Zoric shows where the water came in, where it ran down a shaft to power the turbines, and then where it went through a tunnel to a discharge point at the base of Horseshoe Falls, the largest of Niagara's three cascades.

Marcelo Gruosso, senior director of engineering and operations with the Niagara Parks Commission, has been involved with the project since it was first envisioned in 2017.

"The plant started out with two generators and, by 1924, all 11 were installed, which you see here today," he says, walking through the high-ceilinged building to point out a line of blue, cylindrical generators that fill the space.

"Beside every generator is a 'governor' which regulated the waterflow to a turbine. An air brake in the governor helped adjust the flow. They needed 250 rpm exactly to give them 25 hertz."