To many it's unthinkable, but yes, it has happened on several occasions at the hands of mother nature and once when man intervened.

The first and only time both the American falls and the Horseshoe Falls on the Canadian side fell silent was on the night of March 29, 1848, when an ice jam formed on Lake Erie near Buffalo blocking the water that flows along the Niagara River and over the falls, says Dave Phillips of Environment Canada.

By the next morning a throng of up to 5,000 sightseers had converged on the area to find the American falls had slowed to a dribble and the thundering Canadian falls were stilled, Phillips wrote in a book entitled The Day Niagara Falls Ran Dry, Canadian Weather Facts and Trivia. Some daredevils explored cavities at the bottom of the dry river where they picked up bayonets, muskets, swords, gun barrels, tomahawks and other relics of the War of 1812. Others crossed the river above and below the falls on foot, horseback or by horse and buggy - an historic opportunity, to be sure.

But the waterless river course wasn't seen as an opportunity by everyone: Superstitious people became fearful and anxious and many went to special church services.

The falls wouldn't stay silent for long, though. On the night of March 31 - 30 hours after Mother Nature turned off the tap - balmy weather and shifting winds dislodged the ice and a sudden wall of water surged down the river bed and over the falls, restoring the ever-present Niagara spray and rumble and boom of the falls.

The American falls were also shut off on six other recorded occasions including in 1909, 1936 and 1947, each time when they froze over completely. And in 1969 the American side was silent again - this time at the hands of humans. For seven months the U.S. falls were turned off when the United States Army Corps of Engineers diverted the river to permit repairs to the eroding face of the American Falls.

According to Phillips, Canada's Horseshoe Falls aren't likely to be blocked by ice again. Since 1964 a boom has been positioned at the head of the Niagara River every winter to prevent the formation of ice blockages and to safeguard hydroelectric installations.