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The Netherlands has spent decades building and perfecting one of the most sophisticated water-defense systems in the world. With one quarter of the country below sea level, water remains an ever-present threat, and defending against it — from the rising levels of the North Sea to overflowing rivers — is central to the country’s survival.

But as the rate of sea level rise continues to increase and extreme weather becomes more frequent, experts say the systems that kept the country safe for decades will struggle to keep pace without significant upgrades.

“We live in a bathtub and the bathtub will fill up,” said Co Verdaas, commissioner for the national water defense program, the Delta Program.

“And because we fully trusted centuries of technical water management, we forgot how vulnerable we are,” he told CNN. “We are literally also accessing the limits of what we can do with technical management.”