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.

But the water never stops flowing underneath. That is no accident - the Niagara River is an important source of hydropower, so a long ice boom made from steel catches any icebergs, while ice cutters work around the clock to prevent the falls from jamming up.

If it is sufficiently cold for long enough, an ice bridge forms along the river, connecting the US and Canada.

Visitors enjoyed sledding and drinking on the ice bridge until 1912, when it broke up and three people died - a Canadian couple who became stranded on a piece of ice and a teenager from the US who tried to save them.

Their story is the subject of an online graphic novel, He cock, named for the teenager who died.

Walking anywhere on the ice has been banned ever since. But an exception was made last month for two Canadian ice climbers, Will Gadd, 47, and Sarah Hueniken, 34, who became the first to scale the falls, climbing a 9-meter section of the Horseshoe Falls.

(China Daily 02/26/2015 page1)