After two dams in northeastern Libya failed, thousands of people are dead, thousands more are unaccounted for, and tens of thousands are displaced in the city of Derna and surrounding towns. The dams along the Wadi Derna river valley collapsed amid Storm Daniel, a Mediterranean cyclone that dropped up to 16 inches of rain over parts of the North African country in a single 24-hour period this week. The same record-breaking storm also inundated Greece, Bulgaria and Turkey, causing devastating flooding across the region of those nations before making landfall in Libya.

The scale of the catastrophe in Derna, a city of around 100,000 people, is massive. Yet its underlying causes are not unique. The disaster occurred at the confluence of sociopolitical instability wrought by civil war, a historic storm (likely exacerbated by climate change) and neglected infrastructure: the destroyed dams, first constructed in the 1970s, had reportedly not been maintained since 2002. Similar conditions are replicated in many other places worldwide. In the aftermath of Derna’s dam collapses, experts are calling for renewed attention to the international problem of aging, ill-maintained dams.

Most of the world’s large dams were built in the decades following World War II, between about 1950 and 1985, says Duminda Perera, a civil engineer and risk assessment researcher at the United Nations University Institute for Water, Environment and Health. These dams are important infrastructure that provide reliable drinking water, agricultural irrigation, flood control and electricity to many. Yet dams—like all human-made structures—have a limited life span, degrade over time and require upkeep. On the lower end, “50 years is the reasonable safe age limit,” Perera says; the Derna dams were fast approaching that age. A 2021 U.N. report co-authored by Perera assessed more than 50,000 large dams around the world. He and his co-researchers found that many countries’ dams are, on average, older than age 50 and are at increasing risk of failure. This includes in the U.S., which has the second-highest number of large dams in the world after China and where the average large dam is 65 years old.

The American Society of Civil Engineers regularly issues a “report card” on U.S. infrastructure. In the most recent 2021 assessment, the nation’s dams were given a grade of D. In part, that’s because engineering standards and our understanding of hydrology were far less robust when these dams were built, says Del Shannon, a civil engineer in Colorado and the dam report card’s primary author. Another contributing factor is the mounting, unaddressed structural issues these dams have accrued in recent decades.

Water is powerful. Even concrete dams, such as the eminently recognizable Hoover Dam, are vulnerable to its force over time, says Mark Baker, a retired dam safety engineer who spent more than 30 combined years working on dam safety for the National Park Service and Bureau of Reclamation. Erosion impacts the earth below and around the concrete structure. Often, these dams require reinforcement or new foundation anchors to stay stable. And concrete itself can weaken with exposure to the elements, undergoing subtle chemical reactions that undermine its stability, Baker explains.

Embankment dams—built from materials such as compacted clay, soil and stone—are more common than their concrete counterparts because they’re cheaper, Shannon says. But they’re also even more vulnerable to degradation over time. Embankment dams erode internally as water eats through the center of the structure and pushes supporting material downstream. Without remediation, this results in seepages that can progress into cracks and eventually collapse.

Also, if water outlets aren’t kept properly clear of debris and vegetation, or if a dam and its spillways aren’t large enough to manage the volume in a reservoir, embankment dams are at risk of being overtopped. This is when water pours over a dam’s rim, triggering very rapid erosion of the structure’s front side. In under an hour, Shannon says, water cascading over the front of such a dam can cause collapse. This, he adds, is likely the mechanism by which the clay-and-rock dams in Derna failed—though without more information and a thorough investigation, he emphasizes, this is not yet possible to know for certain.

Regular maintenance, reinforcements and retrofitting can extend a dam’s safe operation well past 100 years and bring a structure up to current standards, Perera and Shannon say. But many dams don’t receive routine repairs and are not aging gracefully. Just making the recommended fixes to most U.S. dams would cost an estimated $157.5 billion dollars, according to a 2023 report from the Association of State Dam Safety Officials. And then there’s the rest of the world, where data on necessary dam rehabilitation and estimated costs are often sparse or difficult to obtain. Yet even when governments or private companies know dam repairs are necessary, they may lack the political will and appropriate funding to take action.

Perera’s 2021 U.N. report identified several dams as dangerous. One example is the Mullaperiyar Dam in the Indian state of Kerala. The structure is more than 125 years old and has visible signs of damage, and it sits at a state border where political relations are tense and in a region where earthquakes are common. If the dam were to fail, an estimated 3.5 million people would be impacted. But the necessary fixes to shore up the structure haven’t yet been made.

In Libya, too, engineers were aware of the Derna dams’ vulnerabilities. A hydrology study of the Wadi Derna Basin published just last year cautioned, “It is clear that the study area is exposed to flood risks.” The study author further wrote (translated from Arabic) that “immediate measures must be taken for regular maintenance of the existing dams, because in the event of a huge flood, the result will be disastrous for the residents of the valley and the city.” If this warning had been heeded, thousands of lives might have been saved.

But it’s not too late to spare other places and people from similar catastrophes. “We should be proactive rather than reactive,” Perera says. Investing in dams, creating early-warning systems and bolstering emergency planning are key, he adds. “It needs to be a global effort,” Perera says.