Picture a waterfall and you see a river, a cliff edge, spray and noise.
The biggest waterfall on Earth is hidden under the sea between Greenland and Iceland. From the surface, there is nothing to see.
It reaches about 3 km below the surface.
Beneath the Denmark Strait, a series of underwater cataracts begins about 600 metres (2,000 feet) below the surface, and the flow descends to water around 3,000 metres (10,000 feet) deep. NOAA calls it the world's largest waterfall.
So how does water fall through water?
It doesn't need open air or an empty gap. It only needs to be denser than the water around it.
It falls because it is denser.
Cold water is denser than warm water. In the Nordic Seas, north of the strait, water loses heat to the cold air above it and grows colder and denser.
As that frigid water flows south through the Denmark Strait, it meets warmer water from the Irminger Sea. The colder water sinks beneath it and follows the seafloor down the slope. This isn't water tumbling through empty space. It is a deep current, pressed against the seabed, pouring downhill underneath lighter water.
NOAA estimates the flow at around 3.5 million cubic metres (more than 123 million cubic feet) of water every second, all of it hidden beneath the waves.
That water doesn't stop at the bottom. Oceanographers consider the Denmark Strait Overflow an important source of dense water feeding the deep, southward-flowing limb of the Atlantic overturning circulation, which helps move water and heat around the ocean.
Stand on a ship in the Denmark Strait and all you would see is the sea. About 600 metres (2,000 feet) below you, the biggest waterfall on Earth begins its plunge.
Sources checked 29 September 2026·3 sources ↓