From WGRZ Channel 2 in Buffalo:
10 stories beneath the raging waters of the Niagara River sits a maze of massive tunnels, each of them 30 feet across. The remnants of the industrial break-through that introduced the world to the power in water.
These tunnels are called tailraces and they are just part of the system of the hydro power generating plants that sit on the Ontario side of Niagara Falls.
There is also a 4+ min video