Sometimes, the North Sea just decides to take a shot at you. Henk Buitjes knows what it’s like.

Once, it happened at night in rough weather. The Dutch shrimp fisher was hauling his nets aboard when out of the corner of his eye he saw the wave. It stood out. Bigger than all the others, it was about to hit the rear of his fishing ship, a beam trawler.

The wave smashed into one of the ship’s two beams, long horizontal poles that hang over the side—they suspend nets in the water. The incredible force of the wave battered the unlucky beam, bowing it, and leaving the net hanging slack, almost impossible to retrieve. That, Buitjes knew, was extremely dangerous. The net might wrap itself around the ship’s propeller or become ensnared on seabed debris.

He and his crew managed to recover it, but Buitjes remembers the rush of adrenalin: “Your whole body is shaking.”

The North Sea is often called treacherous. Just south of the Arctic, it separates Great Britain from Scandinavia, and its southern shores form the northern coasts of France, Belgium, The Netherlands, and Germany. It was once a highway for the Vikings. Today, it is peppered with oil and gas platforms, and offshore wind farms. On social media, eye-popping videos allege that the North Sea is the most dangerous sea in the world—frequently beset by monster waves and ship-pummelling storms.

But people have worked and travelled on this wild body of water for millennia.

What it’s like to sail on the North Sea

Buitjes, 64, and his family have fished on the North Sea for 11 generations.

“My great-grandfather and one of his sons were drowned fishing during a big storm,” says Buitjes. Two of his grandmother’s brothers also died at sea. The uneasy alliance between this family and the North Sea has lasted for 400 years, and counting.

Sailing these waters is safer than it was only a few decades ago, thanks largely to improved weather forecasts, says Buitjes, but there are still things about the North Sea that force mariners to treat it with respect.

It is a shallow sea, with a depth of only around 100 feet in its southern reaches. Sometimes during storms, big waves dredge up sand from the bottom and fling it onto the deck of a ship, says Buitjes. That can make a vessel unstable. Over the years, ships lost in tempests have piled up on the seabed, themselves becoming hazards to vessels that drag nets around. One area in particular, the west coast of Jutland—a peninsula that encompasses much of Denmark and part of Germany—is famous because so many ships have foundered after being smashed against it. With no natural harbours to shelter in, it has long been nicknamed the “Iron Coast”.

Part of the trouble with the North Sea, says Buitjes, is that the waves tend to be short. They come again and again in quick succession and can push a ship around like an unhappy cork. Even though these waves might not be very high, around six feet, they can still cause a vessel to feel extremely unsteady.

This choppiness is a well-known characteristic of the North Sea, says Lucy Bricheno, a coastal oceanographer at the U.K.’s National Oceanography Centre. It’s partly because the sea is enclosed by land on many sides—waves don’t have much space to spread out and build giant peaks.

When the tail end of a hurricane, perhaps one that has hit the eastern U.S., ends up on the other side of the Atlantic, it can ravage the North Sea, whipping these waters into a squally frenzy. Sometimes, the waves become so feisty that their white caps are visible from space.

According to Bricheno, modelling suggests the North Sea is actually set to become calmer, on average, under climate change. However, the same models indicate that when severe storms do occur, they could be worse than in the past. It’s a trend that appears to be emerging already.

“We can’t say for sure it’s definitely stormier but, what we can say, is the storms we are having in northwest Europe are getting more severe,” says Bricheno.

Why the North Sea has such a dangerous reputation

Despite all this, Bricheno notes that the North Sea is far from the stormiest body of water in the world. The Southern Ocean, for example, is largely unbroken by land, which lets gigantic waves build and build. Waves 25 to 30 feet tall are standard there, she says. In 2018, a buoy detected a gargantuan wave of 70 feet. However, ship traffic is relatively low in the Southern Ocean.

Perhaps we think of the North Sea as treacherous somewhat ironically—because it is so well-traversed. Many have experienced its foulest moods and nowadays can even record what they’ve seen on their smartphones, then share the footage online.

The Vikings, 1,200 years ago, were able to find their way around the North Sea. They likely used intricate knowledge of sea currents and the weather, as well as visible landmarks, on these epic voyages.

“It’s rather like having your own motorway for ravaging and pillaging,” says Michael Pye, author of The Edge of the World: How the North Sea Made Us Who We Are. Victims of Viking assaults around Europe probably came to dread the North Sea and those who sailed across it.

Even during the medieval period, though, there were much more terrifying seas, says Pye: “Down the west coast of Africa, for example, there was cape after cape beyond which you could not go.”

And back then, the Iron Coast of Jutland was probably safer than it is today, suggests Thorbjørn Thaarup, curator at the Maritime Museum of Denmark. There were inlets and small bays where you could gather your companions and vessels together before setting off. That appears to have changed dramatically in 1362, with an immense flood known as “The Great Drowning” on the west coast of Jutland. Up to 10,000 people died and huge swathes of land, including whole villages, were swallowed up by the sea forever.

North Sea tides are unusually large, says Bricheno. This can make storm surges especially dangerous if a surge coincides with high-tide. Records of such catastrophes go back hundreds of years. One recent example was in 2013, when a surge at high-tide caused two deaths and 1,700 homes to be flooded in England.

The North Sea lets you know when it is angry, says Dorthe Nors, author of A Line in the World: A Year on the North Sea Coast. Her home, in the west of Denmark, is about two and a half miles from the shore. But she can still hear the sea when a storm is up.

“It’s like somebody is screaming, like a roar,” she says. When winds break trees near the coast, the snapping trunks sound like gunshots. And sand from the beaches can get tossed up onto coastal roads, blocking traffic.

Buitjes tells stories of various accidents he has heard about in the North Sea, affecting small fishing vessels and large container ships alike. Whenever you sail here, you have to keep your wits about you, he stresses. These are some of the most powerful forces in nature.

“Be aware of the tininess of yourself,” he advises. “When you are aware of that, you will survive.”