We should acknowledge upfront that Maarten de Groot is a composite figure—a historically grounded invention built from archival records of the Zuiderzee Works engineering team, period correspondence, and technical reports from the 1920s. Think of this as what you might have heard if you'd bought a junior engineer a jenever in a Rotterdam café in 1932, right after the Afsluitdijk dam sealed off the Zuiderzee from the North Sea. The calculations were real. The stakes were real. The uncertainty was very, very real.
We meet in what used to be a fishing port but is now, improbably, thirty-two kilometers from the sea. The Afsluitdijk—the "closing dike"—has been finished for three months, and the Zuiderzee is now the IJsselmeer, a freshwater lake where salt water used to rage. De Groot looks tired in the way of someone who spent sixteen years wondering if his math was wrong.
You were how old when the dikes broke in 1916?
Maarten: Twenty-four. Fresh out of Delft, working for the provincial water board. I was supposed to be checking routine maintenance reports. You know, which dikes needed reinforcement, where we should add clay.
Then on January 14th, I'm reading reports about dikes that don't exist anymore. Just gone. Swept away.
The Waterland dike failed for 1.5 kilometers. You have to understand, these weren't small village dikes. These were the main defense. And they just dissolved.1
What do you remember about the days after?
Maarten: Chaos. Purmerend underwater. Hundreds dead—we never got a final count because some bodies washed out to sea.
And this immediate, sick realization that it would happen again. Not if. When.
But here's what nobody tells you about disasters: they create permission. Lely had been proposing his Zuiderzee plan since 1891, and for twenty-five years everyone said it was too expensive, too ambitious, too risky. Then the flood happens, and suddenly the risk is not building it.2
So you joined Lely's team?
Maarten: 1918, right after the Zuiderzee Act passed. They needed engineers who could do the actual calculations. Not the grand vision stuff, but the tedious work of figuring out which parts of the Zuiderzee could actually become farmland and which parts we should leave as lake.
Lely was brilliant, but he was also a politician by then. Minister of Transport. He needed people who could tell him what was possible, not just what was desirable.
And what was possible?
Maarten: [Laughs] Less than everyone wanted.
See, the popular imagination was drain the whole thing. The entire Zuiderzee becomes land. We'd gain back everything we'd lost to the sea over seven centuries—we'd lost 1.4 million acres since 1200, and people wanted it back.3
But I spent six months analyzing the test drilling data from the Zuiderzeevereeniging, and the reality was brutal. About three-quarters of the Zuiderzee had clay soil that could be farmland. The rest? Sandy bottom. Useless for agriculture. You'd spend a fortune draining it to create land nobody could farm.
So we had to make this calculation: How much sea can we afford to steal?
That's a hell of a question.
Maarten: It was worse than that. Because it wasn't just about money—though the costs were staggering. It was about time.
Every year we delayed, more floods could happen. But if we tried to do too much and the project failed, we'd have wasted everything.
Lely wanted to be bold but not stupid. So we designed for about 500,000 acres of good clay land, and we'd leave the sandy parts as the IJsselmeer.4 Smaller lake, but still a lake.
I remember the meeting where we finalized that decision. One of the senior engineers—I won't name him, he's still alive—he kept pushing for the full drainage. "We're already building a 32-kilometer dam across open sea," he said. "Why stop there?"
And Lely just looked at him and said, "Because I want this to work."
The Afsluitdijk is absurd. How do you build a dam across open ocean?
Maarten: Carefully. And by accepting that you're going to be wrong about some things.
We started with the Amsteldiepdijk in 1920—much smaller, only 2.5 kilometers—as a test case. Took four years. Every mistake we made there, we learned from.5 How to handle tidal currents. How to sink the final closing gap without the whole thing washing away. How to prevent the dam from settling unevenly.
Then we started the Afsluitdijk in 1927. Thirty-two kilometers. We worked from both ends—Den Oever and Friesland—building toward the middle.
The tidal currents in the closing gap got faster and faster as the opening narrowed. By the end, we were dumping material into water moving at terrifying speeds, just hoping it would hold.
May 28, 1932. We closed the gap. I was there. I watched the last loads of clay and stone go in, and I thought: "This either works or we've just created the most expensive artificial reef in history."
Did you believe it would work?
Maarten: [Long pause] I believed our calculations were sound. I believed we'd done everything we could.
But there's a difference between believing your math and believing in the hubris of it all.
We were changing the map. Permanently. The Zuiderzee had been there for centuries—it formed after massive floods in the 1200s—and we were just removing it.6 Creating provinces that didn't exist. Committing our children and grandchildren to maintaining this infrastructure forever, because if they don't, the sea takes it all back.
That's what keeps me up at night. Not whether the dam will hold—it will, the engineering is solid. But whether we've locked future generations into a bargain they didn't agree to.
What bargain?
Maarten: Maintenance. Forever.
The pumping stations that drain the polders, the dikes that hold back the IJsselmeer, the Afsluitdijk itself—it all needs constant work. Stop maintaining it, and within a generation, the sea reclaims everything.
We've created land that only exists because we will it to exist. That's not normal land. That's a promise. Or maybe a threat. I'm still not sure which.
But you're also creating food security. Agricultural land. The whole point was flood protection and feeding people.
Maarten: Right. The First World War food shortages made everyone desperate for arable land. That's why the Act passed in 1918—safety and food, both at once.7
And it's working. The Wieringermeer polder is already being drained. In a few years, people will be farming land that was underwater when I was born. That's extraordinary.
But here's what I think about now: We made these choices based on 1916 conditions. What if conditions change? What if the sea level rises? What if the storms get worse?
We've designed for the threats we know, but we've also committed to a system that's incredibly rigid. You can't just move a 32-kilometer dam.
Do you regret it?
Maarten: No. God, no. The 1916 flood was a nightmare. If we'd done nothing, it would have happened again, and again, and people would have died. We had to act.
But I think we should be honest about what we've done. We haven't "conquered" the sea. We've made a very expensive, very complicated deal with it. As long as we keep our end—the maintenance, the pumping, the vigilance—the sea stays back.
But it's always there. Waiting.
He looks out at what used to be the Zuiderzee and is now placid lake water.
Maarten: Sometimes I wonder what future engineers will think of us. Whether they'll see the Afsluitdijk as a triumph or as the moment we got overconfident. Whether they'll curse us for locking them into this endless maintenance, or thank us for buying them time.
I guess they'll probably feel both. That's usually how these things go.