The world’s largest waterfall is not found on land and it cannot be seen from the surface. Hidden beneath the Atlantic Ocean between Greenland and Iceland, the Denmark Strait cataract is a massive underwater current in which cold, dense Arctic water plunges down the seafloor slope beneath warmer ocean water.
The submerged cataract spans an estimated vertical extent of about 3.5 kilometres, or 11,500 feet roughly 3.5 times the height of Angel Falls, the world’s tallest waterfall on land.
Unlike a conventional waterfall, however, the Denmark Strait cataract does not involve water falling through air. Instead, differences in temperature and salinity make the northern water denser. As this cold, heavy water crosses an underwater sill in the Denmark Strait, gravity pulls it down the Greenland continental slope.
The cataract begins near a submerged sill roughly 600 metres below sea level and extends into the deep North Atlantic. Rather than forming a single vertical cascade, the dense water spreads across the seabed, accelerates through channels and mixes with surrounding ocean water as it descends.
Scientists estimate that the flow transports millions of cubic metres of water every second. NOAA has estimated the flow at more than 3.5 million cubic metres per second, equivalent to about 3.5 sverdrups.
The enormous movement remains invisible from the surface because the entire process takes place underwater. There is no exposed cliff, falling sheet of water or spray.
The phenomenon is driven by density.
Cold water is generally denser than warm water, while higher salinity also increases seawater density. In the Nordic Seas, extremely cold water becomes dense enough to sink beneath warmer surrounding water.
When this dense water reaches the Denmark Strait, it flows over the underwater sill and down the Greenland slope, effectively behaving like a giant submerged waterfall.
As it descends, the water entrains surrounding seawater and changes in volume and properties. Scientists therefore use the term “waterfall” to describe the scale and downward movement of the current, rather than a conventional free-falling cascade.
The Denmark Strait overflow is more than an underwater curiosity. It plays an important role in the Atlantic Meridional Overturning Circulation (AMOC), the vast system of ocean currents that transports heat through the Atlantic.
The dense water flowing southward from the Nordic Seas contributes to the deep limb of this circulation. Together with other ocean overflows, winds, mixing and deep-water formation, it helps drive the large-scale movement of heat and water through the Atlantic.
Researchers cannot simply observe the cataract from a ship. Instead, they use seabed moorings, temperature and salinity sensors, pressure instruments and acoustic Doppler current profilers to measure the movement of water at depth.
Research vessels also collect measurements throughout the water column, allowing scientists to map the dense overflow as it moves down the continental slope.
The Denmark Strait cataract is therefore a reminder that some of Earth’s most powerful natural phenomena are hidden from view. Far beneath an apparently ordinary stretch of Atlantic Ocean, billions of tonnes of cold, dense water are continuously moving through the deep ocean helping power one of the planet’s major circulation systems.