Pieces of ice flow over the Canadian Horseshoe Falls in Niagara Falls, Ontario, Canada, Feb 19, 2015. [Photo/IC]

It's a stunning site: Niagara Falls, the world's best-known waterfall, apparently frozen in place.

The icy spectacle, brought on by weeks of severe cold, has drawn a steady flow of intrepid tourists. But are the falls really frozen? Not exactly.

Here's what happens when the mighty Niagara River appears to form a crystalline cascade:

For the second consecutive winter, frigid weather in western New York state and lower Ontario in Canada has frozen vast sections of the United States side of Niagara Falls, known as the American Falls, and the Canadian side, known as the Horseshoe Falls because of its shape.