Largest underwater waterfall: Denmark Strait cataract
This plays around with the definition of a waterfall, but keep your mind open because this is is pretty amazing.The Denmark Strait, in the Atlantic Ocean between Greenland and Iceland, is an underground waterfall that tumbles almost 11,500 feet and carries 175 million cubic feet of water per second.
The reason it exists is due to temperature differences in the water on either side of the strait. Cold water is more dense than warm water. And the eastern side of the strait is a lot colder than the western side. So when the waters meet, the cold water sinks below the warmer water, creating a strong downward flow of water — one that can be (and is) considered a waterfall.
There are other areas of the ocean where the same thing happens, but the Denmark Straight is hands down the largest of them.