Niagara Falls is made up of three waterfalls from the Niagara River. In 1969, the US Army Corps of Engineers stopped the flow of water over one of them, American Falls, to study the stability of the rock underneath. For six months, tourists could walk over the rock bed where the falls had been. Ultimately, the Corps decided that stabilizing the rock bed would not be worth the expense.
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(Image credit: Flickr user Russ Glasson)
Previously: The falls also stopped flowing in 1848 for natural reasons, and froze completely over in 1911.