Stand beside a waterfall and you can see where the water goes. Earth’s largest waterfall offers no such view: it lies beneath the Denmark Strait, between Greenland and Iceland. Called the Denmark Strait cataract, it is an enormous flow of seawater descending along the ocean floor—not a river plunging through open air.
How can a waterfall exist underwater?
The ocean may look like one continuous body of water, but its waters differ in temperature and density. In the Denmark Strait, frigid water flowing south from the Nordic Seas meets warmer water. The colder, denser water sinks beneath it and spills over a submerged ridge, continuing down the seafloor.
That is why waterfall is a useful description, though it can also give the wrong picture. There is no visible curtain of water or spray rising above a plunge pool. From a boat, you would not see the cataract at all; scientists detect what is happening beneath the surface with instruments.
Just how big is it?
NOAA estimates that the seafloor descent associated with the cataract is about 11,500 feet (3.5 kilometers) and that more than 123 million cubic feet (3.5 million cubic meters) of water flows downward each second. The height figure describes the underwater drop; it does not mean the current falls freely through an empty space for that distance.
Its importance goes beyond its size. Water passing through the strait helps feed the deep return flow of Atlantic circulation. That larger system moves heat and other materials through the ocean, linking this hidden descent to processes far beyond the waters between Greenland and Iceland.
The remarkable part is easy to miss: one of Earth’s greatest flows is happening continuously below the waves, out of sight even when the sea above it appears calm.