The largest waterfall in the world is Niagara Falls.
Why it is true
The largest waterfall in the world is the Denmark Strait cataract, an underwater waterfall between Greenland and Iceland. Cold, dense water falls about 3,500 metres, but the phenomenon occurs beneath the ocean surface.
Source: NOAA
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This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.