Where’s the world’s largest waterfall? Nope, it’s not Victoria Falls, as majestic as that is; neither is it Angel Falls, famously so tall that much of the water doesn’t even reach the bottom. In fact, it’s the Denmark Strait cataract – and while you won’t have seen any photos of it, and maybe haven’t even heard its name before, it’s actually larger than both those more famous falls put together.

The Denmark Strait cataract: the waterfall you’ve never heard of

The Denmark Strait cataract – the word also refers to a big waterfall, from the Greek for “downward rushing” – is way larger than any waterfall you’ve seen any pictures of before. It’s 3,505 meters (11,500 feet) tall, all told – that’s more than 3.5 kilometers, or nearly 2.2 miles – even if the actual “fall” bit is only (“only”) about 2,012 meters (6,600 feet) of that height. 

It’s roughly 480 kilometers (300 miles) across, and more than that in width, and the output it sends into the Atlantic Ocean is between 20 and 40 times the sum of all the water that comes to it from rivers. In fact, let’s put it in more mind-boggling terms: the amount of liquid flowing through the Denmark Strait cataract is equivalent to about one-and-a-half Great Pyramids of Giza-worth of water… every second.

It’s all very impressive – so why is it so obscure? Well, it’s not exactly a tourist destination: located on the edge of the Arctic Circle between Iceland and Greenland, you’re more likely to get frostbite than a tan if you visit Denmark Strait. Aside from frequent icebergs and fishing voyages, the most notable recent presence in the area was a bunch of Nazis in May 1941.

But the biggest obstacle to visiting this world-beating waterfall is simple: it’s underwater.

“Rivers flowing over Earth’s gorges create waterfalls that are natural wonders, drawing millions of visitors to their breathtaking beauty, grandeur, and power,” notes NOAA (the National Oceanic and Atmospheric Administration). “But no waterfall is larger or more powerful than those that lie beneath the ocean, cascading over immense cataracts hidden from our view.”

An underwater waterfall??

Now, we know what you’re thinking: how can a waterfall exist underwater? Surely that screws with the whole waterfall, you know, gig? But it’s actually simple physics: “cold water is denser than warm water, and in the Denmark Strait, southward-flowing frigid water from the Nordic Seas meets warmer water from the Irminger Sea,” NOAA explains. “The cold, dense water quickly sinks below the warmer water and flows over the huge drop in the ocean floor, creating a downward flow estimated at well over 123 million cubic feet [3.5 million cubic meters] per second.” 

And yet, perhaps surprisingly, if you somehow could go and look directly at it, it probably wouldn’t be all that impressive: it’s so wide and broad that the water only reaches speeds of around 50 centimeters (20 inches) per second – that’s 1.8 kilometers per hour, 1.2 miles per hour, or about as fast as a toddler taking their first independent steps.

“It’s probably dropping about 2,000 meters [6,560 feet] vertically down into the depths of the Atlantic Ocean,” Mike Clare, leader of marine geosystems at the UK’s National Oceanography Centre in Southampton, told Live Science back in April. “But [it’s] over quite a big distance of something like 500 to 600 kilometers [310 to 370 miles].”

In other words, “it looks like a relatively low-gradient slope,” he explained. “If you were down there, you probably wouldn’t notice a whole heap going on.”

Which does make you wonder… 

How was this even discovered? 

Here’s the thing: the Denmark Strait cataract may not look very impressive, but it’s vitally important for the local marine ecosystem. It’s part of the system of ocean currents known as the Atlantic Meridional Overturning Circulation, or AMOC – one part of “a large ‘global conveyor belt’,” explains NOAA, which “circulates water from north to south and back in a long cycle within the Atlantic Ocean.” 

“The circulation process begins as warm water near the surface moves toward the poles […] where it cools and forms sea ice,” the agency explains. “As this ice forms, salt is left behind in the ocean water. Due to the large amount of salt in the water, it becomes denser, sinks down, and is carried southwards in the depths below. Eventually, the water gets pulled back up towards the surface and warms up in a process called upwelling, completing the cycle.”

It brings not just cooler waters and climatic stabilization to the waters on its journey, but also oxygen, nutrients, and organic matter crucial to support marine life – so it was with good reason that geologists back in the 60s went over to the Strait to see what was going on. 

They recognized what they found as being so important that they launched the landmark – watermark? – Overflow ’73 program, in which the water speed, direction, and temperature were measured every 15 minutes for a full month and thorough hydrographic surveys of the area were conducted. For the first time, the deep waters of the cataract had been mapped, adding vital bonus questions to trivia nights around the world.

Will the Denmark Strait always be the biggest?

Unfortunately, like – well, just about every important geological or climatic system, the AMOC is under threat from climate change. “Even though the whole process is slow on its own, there is some evidence that the AMOC is slowing down further,” warns NOAA. “Whether or not it will continue to slow or stop circulating completely remains uncertain.”

If it were to slow too far, or even stop entirely, the effects could be nothing short of devastating. “If the AMOC does continue to slow down […] freshwater from melting ice at the poles would shift the rain belt in South Africa, causing droughts for millions of people,” says NOAA. “It would also cause sea level rise across the U.S. East Coast.”

And, over in between Greenland and Iceland, the Denmark Strait cataract will suffer too. We’ve known since at least the 1980s that it is particularly vulnerable to climate change; if the AMOC dies, then the cataract “will decrease in density and it will stop,” Anna Sanchez Vidal, a professor of marine science at the University of Barcelona in Spain, told Live Science. 

And what then? We’ll just have boring old Victoria and Angel Falls to vie for the title of biggest waterfall. 

Well… at least until they get destroyed as well.