Niagara Falls © Judith Duk
The Niagara Falls straddle the United States and Canadian border, 340 miles (547km) northwest of New York City. They are one of the most popular natural attractions in the country, attracting more than 20 million tourists a year. The river plunges over a cliff of dolostone and shale, forming the second largest waterfall on earth after Victoria Falls in southern Africa. While best appreciated via a boat trip, there are a number of different tours available which will probably touch on the numerous daredevils that have braved its drop. Many have gone over in barrels, while a tightrope crossing in 1859 has inspired scores of tightrope walkers to do the same.