There’s a kind of wave in the ocean that can dwarf anything at Nazaré, home to what most people think of as the world’s biggest waves. You won’t see it break, won’t hear it roar, and you could sail straight over it without realizing a monster just passed beneath you.
Welcome to the strange world of internal waves: enormous, nearly invisible swells that move through the ocean itself, rather than across its surface.
In a recent video, narrator James Stewart describes the phenomenon (“some as tall as the Empire State Building”) that form between Taiwan and the Philippines before rolling across the South China Sea. Watch below, and learn more about these mysterious monsters.
“From the surface, you would have no idea they were there,” he explains.
“These are the largest waves produced in the world’s oceans, with up to 10,000 times the amount of turbulence typically found in the open ocean,” he continues. “Every single day these waves, some as tall as the Empire State Building and up to 200 km wide, form in the waters between Taiwan and the Philippines, rolling across the South China Sea.”
So, how can a thousand-foot wave hide underwater?
Unlike familiar wind-generated swells, internal waves travel along boundaries between layers of seawater with different densities. Warm, relatively light water often sits above colder, denser water. When tides push those layers over underwater mountains and ridges, their boundary rises and falls in giant undulations.
That enormous height describes the vertical movement of underwater layers, not a conventional wave face rising above the sea.
As Woods Hole Oceanographic Institution explains, it takes far less energy to move water into another layer of water than to lift it into the air.
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“Underwater, you get more bang for your energetic buck,” the institution’s report explains.
These aren’t theoretical monsters. A study published in Nature documented breaking internal waves more than 650 feet tall in the Luzon Strait, generating turbulence over 10,000 times greater than typically found in the open ocean.
They’re found elsewhere, too, from the Andaman Sea to Cape Cod Bay. Their telltale surface signatures are often nothing more than alternating bands of smooth and rough water, visible in satellite imagery.
And they can actually break, just not in the way surfers recognize. Researchers at UC Irvine explain that as internal waves approach shallower continental shelves, they steepen, collapse, and mix deep water upward.
That mixing transports nutrients, affects marine life, and helps redistribute heat throughout the ocean.
The biggest waves on Earth may be the ones nobody ever sees.
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