The largest waterfall on Earth flows under the Atlantic Ocean. You could sail over it between Greenland and Iceland and miss the entire thing. Far beneath the surface, cold, heavy seawater moves over an underwater ridge and down into a much deeper basin.
It’s called the Denmark Strait cataract, and the National Oceanic and Atmospheric Administration (NOAA) identifies it as the world’s largest waterfall. The name sounds like a river tumbling off a cliff. What scientists measure here is a vast, descending ocean current.
How can one part of the ocean fall through another?
Temperature and salt affect how dense seawater becomes. When water in the far North Atlantic cools, it can grow dense enough to sink below warmer water around it. That gives gravity a way to separate water masses even though both are seawater.
In the Denmark Strait, a submerged ridge sits between the Nordic Seas and the deeper Irminger Sea. Cold water arriving from the north passes across this underwater barrier and follows the seafloor downslope. The Woods Hole Oceanographic Institution (WHOI) describes the overflow as a major gateway for deep water moving south through the Atlantic.
Picture an enormous slow-moving stream following the contours of the bottom. There is no exposed precipice where water disappears into open air. The movement happens entirely beneath the waves.
How big is the Denmark Strait waterfall?
NOAA’s illustrated estimate gives the cataract a height of approximately 11,500 feet (3.5 kilometers). For comparison, the agency lists the tallest waterfall on land at about 3,212 feet (0.98 kilometer).
The measurement needs context. NOAA’s text describes a series of undersea cataracts that begin about 2,000 feet beneath the surface and lead toward water depths of roughly 10,000 feet south of the strait. Its 11,500-foot infographic figure is an approximate height for the vast system, rather than the measured face of one free-falling cascade.
The amount of moving water is extraordinary, too. NOAA estimates downward transport at more than 3.5 million cubic meters per second. That’s a quantity measured across a broad current, not a narrow spout an observer could stand beside.
Why do passing ships never see it?
At the surface, this part of the North Atlantic looks like open ocean. The denser flow stays below lighter water and moves along the seabed. NOAA says the turbulence produced by this underwater cataract cannot be detected at the surface without scientific instruments.
Oceanographers have to measure the current itself. During work in and around the Irminger Sea, WHOI researchers used equipment to record water temperature, salinity and current speed at different depths. Their field account explains how sensors attached to moorings recorded changing conditions beneath the ships.
That’s how an immense flow can be studied even when there is no visible waterfall to photograph.
Why oceanographers care about the hidden current
The Denmark Strait overflow supplies cold deep water that continues south in the Atlantic. It forms part of the broader exchange in which warmer waters move north near the surface and colder waters return at depth. This circulation helps transport heat around the ocean.
NOAA’s explanation of deep-ocean water and WHOI’s description of the undersea overflow both connect the density-driven motion to that larger circulation. Researchers study how much water crosses the ridge, where it goes, and how its flow changes over time.
For an entirely different cold-region curiosity involving moving water, Antarctica’s Blood Falls gets its red color from iron-rich brine emerging from a glacier. The Denmark Strait cataract has no dramatic color to advertise its presence. Its scale is revealed by measurements taken far below the surface.