You might have heard of Victoria Falls, Iguazu Falls, Angel Falls and the Niagara Falls but did you know that the world’s largest waterfall is actually underwater?
Deep below the surface of the waves, the seafloor isn’t flat – far from it. There are underwater mountains (seamounts), canyons, trenches (including the deepest part of the ocean in the Mariana Trench) and, yes, waterfalls.
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The biggest of these is located in the waters that separate Iceland and Greenland. Called as the Denmark Strait Cataract, it’s the largest waterfall on the planet.
“The cold water falls more than 3.5 km, at least three times the height of Angel Falls,” writes Guinness World Records on its website. “This underwater waterfall carries around five million cubic metres of water per second.”
For comparison, around 168,000 cubic metres of water go over Niagara’s Horseshoe Falls every minute in summer, according to Niagara Falls Tourism.
But how can you have a waterfall underwater? It’s all to do with temperature. Cooler water is denser and so sinks below warmer water. So, when the icy Arctic water coming from Greenland meets a steep drop in the ocean floor, it plummets over the precipice – just like a waterfall on land – and sinks under the warmer Atlantic water from the Iceland side.
“The poles are the regions where most of the dense water masses – generated by the formation of sea ice at the surface – eventually reach the global ocean floor,” says David Amblas, a marine geoscientist in the Department of Earth and Ocean Dynamics at the University of Barcelona (UB) in a news release. “The polar areas are like the heart of the oceanic circulatory system: they pump cold, dense water into the great oceanic troughs through the heartbeats made by overflows of dense water.”
The Denmark Strait Cataract was discovered relatively recently, in 1989.
In a 2020 Royal Institution lecture, Helen Czerski, an oceanographer and physicist at University College London highlights how mind-blowing it is that this incredible phenomenon is happening out of sight, saying: “Sailors have been crossing from Iceland to Greenland for centuries and they’ve never known that this enormous waterfall was just underneath them.”
Although we can’t see this immense subsea waterfall, our lives are impacted by its effects. Researchers believe the water flow from the Denmark Strait Cataract influences a system known as the Atlantic Meridional Overturning Circulation (AMOC for short).
The AMOC is a conveyor-belt-like ring of ocean currents that drive water around the Atlantic Ocean: warm water is pushed north where it cools and returns south again.
“This global process makes sure that the world’s oceans are continually mixed, and that heat and energy are distributed around the earth,” explains the Met Office on its website. “This, in turn, contributes to the climate we experience today.”
Scientists are worried about how warming waters will affect these systems. If Arctic waters warm, the cataract’s flow could become weaker, which would cause the AMOC to slow down.
According to the National Oceanography Centre, the ripple effect could result in sea level rise and changes to regional climates. Also, they say, it “might change rainfall patterns, storm tracks, and the frequency of extreme weather events.”
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