When the Xe-Pian Xe-Namnoy dam burst in southern Laos on July 23, it released 5 billion cubic meters of water that killed at least 35 people, displaced more than 6,000 and swamped 18,000 acres of land that stretched into neighboring Cambodia.

It also let loose an unknown amount of unexploded ordnance—a remnant of the Vietnam War.

Between 1964 and 1973, US forces dropped millions of bombs on the Ho Chi Minh Trail, which ran through the Laotian province of Attapeu, near the Vietnam border. They saturated the land with everything from incendiary devices designed to spark fires, big bombs of 1,000 pounds or more and cluster submunitions packed into casings that opened mid-air, scattering millions of little “bombies” the size of baseballs across the land.

Many of these didn’t explode at the time, and they remain in the ground, still volatile half a century later.

Laos is the most heavily bombed country in the world, per capita. Today, 94 percent of all villages in Attapeu contain unexploded ordinance (UXO)—including the villages most affected by flooding.

“This situation is extremely critical,” says Peter Menzel, a fluid mechanics expert at the University of Rostock in Germany, who studies the effects of water currents on UXO. Floodwaters have the force to move ordnance, he says, threatening lives and exacerbating the risks of rebuilding. “UXO can end up at positions where you never would expect them to be.” Areas that previously had been cleared of bombs may no longer be safe.

UXO Lao, the national clearance agency, reportedly has more than 100 staff on the ground in Attapeu, searching for and removing bombs before construction begins on temporary homes for residents who were evacuated.

But flooding is a problem that extends beyond Attapeu, too: last month’s Tropical Storm Son-Tinh, which contributed to the dam collapse, brought particularly heavy rains that elevated rivers, generated landslides and swamped some 373 villages across 10 provinces.

As The Asia Foundation reported this week, Laos is susceptible to catastrophic weather that has prompted public awareness efforts about disaster-risk reduction. But UXO presents an added risk that requires specialized responses from trained professionals.

This is always true when forces of nature or human intervention disturb UXO.

Unexploded bombs packed tightly in the ground can remain so for years without incident. But movement can cause them to explode.

Mee Ly knows this, which is why she is so scared. She has spent her life growing corn and rice around her home in northern Laos, more than 400 miles from Attapeu. The Xe-Pian Xe-Namnoy floodwaters pose no threat to her family, yet every year she worries about seasonal monsoons, which last through September. “We’re very afraid when it’s the rainy season like this,” she said a month before the Attapeu flooding. As heavy rains move soil and sediment, “the bombs are coming up.”

That’s what Mee Ly observes in a downpour; the flood of a dam collapse is magnitudes larger in terms of uncertainty and risk.

In fact, there are many ways bombs can move from one spot to another: a heavy rain that triggers a landslide; a dam that breaks and unleashes a flood; a villager who finds a bomb and tosses it in the woods to keep nearby children safe (Mee Ly’s husband did that when their daughter found a bombie on the fence line last year). That’s why clearance groups guarantee a site’s safety only at the moment they’ve finished clearing, and only to the depth at which they’ve cleared (that depth depends on the intended use of the land, but the standard for rice fields in Laos is 25 cm, or 9.8 inches). “What happens after the handover is out of a clearance organization’s control,” says Stuart Broome, an Australian UXO expert who conducted clearance in Laos and Vietnam for many years.

It’s one thing for a person to move a single bombie from one spot to another. But a collapsed dam can create a flood of high velocity that “can easily mobilize larger stones, maybe cars and of course UXO,” Menzel says. He recommends closing the area to people “until the situation is cleared and the area is definitely safe.” To him, that means performing a UXO survey with the combined expertise of clearance technicians, geologist and specialists in fluid mechanics and sediment transport.

Some of that work was in progress when the dam broke: UXO surveys have been ongoing for years in Laos. With an estimated 80 million unexploded bombs left at the end of war, and only about 1 percent of bombies removed in the decades since, clearance is a long-term mission. Thousands of Laotians work daily, scouring the land for signs of metal and dangers to remove. The Obama administration pledged an unprecedented $90 million for clearance and survey to mark the greatest risk areas across the country, including in Attapeu.

But now, assurances will be replaced with fears, says Jim Harris, founder of the American nonprofit We Help War Victims. He spends months each year working with Lao bomb clearance teams. Attapeu villagers living on previously cleared land “have no way of knowing fully that their land is still safe.” Many of the tasks entailed in reconstruction—chopping wood and uprooting trees, digging holes and pounding posts into the ground, moving debris with heavy machines—are dangerous in any area with bombs. This work will compound the risks of daily life. “Most of the people are out in the environment struggling to make a subsistence living,” Harris says. “If they’re not farming, they’re foraging. If they’re not foraging, they’re hunting. If they’re not hunting, they’re fishing. They have a lot of outdoor activities that take them through land that has never been cleared. But now, even the certainty that they once had is gone.”

There is a deep divide between fear and security in Laos, and UXO is a defining factor.

For the last several years, a villager named Soun has lived in relative safety near another dam, in Khammouane province, north of Attapeu. When that dam was constructed, Soun’s family was moved to a new home on cleared land. Ever since, he has not worried about bombs around his house. “This is very good,” he said a few weeks before the Xe-Pian Xe-Namnoy disaster. “We’ve been safe.”

But then one day in June, Soun’s neighbor found a 500-pound bomb. He hauled it back to the village via motorbike, but the villagers scolded him, telling him it was dangerous. So he took it away. “Now I don’t know where he put it,” Soun said a week after the incident. And everyone was worried.

Soun and his wife suspected the bomb had surfaced in the rise and fall of water in the nearby reservoir: one bomb found by one villager in the movement of water behind a dam that remained intact.

Imagine, then, the havoc of an accidental deluge that turns water, earth and bombs into a cascade of unknowns, engulfing thousands of homes.

No one knows where the bombs will end up. But villagers like Soun and Mee Ly know exactly how the residents of Attapeu will feel as they attempt to rebuild amid fields and forests of uncertainty and risk.