Masses of ice forms around the Canadian Horseshoe Falls in Niagara Falls, Ontario, Canada, Feb 19, 2015. [Photo/IC]

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.