Land mines that were moved by floods after the Kakhovka Dam broke this week could kill thousands of people trying to leave their homes and pose a serious threat to people in southern Ukraine for decades to come.

Fears are also growing about the risks of waterborne diseases and dangerous chemicals that could poison not only the floods but also the Black Sea, which is an important waterway. This is why both sides of the war are sending warnings.

One major mine-clearing charity says that thousands of unexploded mines could have been washed away from areas that had already been marked. This could have made Ukraine the worst war for unexploded ordnance since 1945.

The Halo Trust, a land mine clearance NGO based in Washington, has done mine clearance operations all over the world. However, Mike Newton, who is in charge of Halo’s operations in Europe and has worked in Mozambique, Cambodia, and Somalia, says that none of those challenges are as hard as the current one.

He told NBC News, “This is the worst contamination anywhere since the Second World War. The Balkans look like a picnic in comparison.”

Halo has done a lot of research on anti-tank mines in the area around Mykolaiv, which is about 60 miles northwest of Kherson and 30 miles from the Dnieper River. In November 2022, the Russians were kicked out of the city.

In the last month, its teams in the Mykolaiv area found more than 5,000 anti-tank mines, 464 of which were near rivers. Three of these minefields are completely submerged now.

Newton said, “That’s just a taste of what we might find in other places.”

During Russia’s 15-month invasion of Ukraine, when the dam broke, it was called a humanitarian and environmental disaster that had never happened before. So far, thousands of people have been removed, and Russia has asked the rest of the world for help.

Oleksandr Prokudin, who is in charge of the troops in the Kherson region, said on Friday that 2,300 people have been saved from 3,600 homes in 32 towns. He said on Telegram that emergency services were even able to save 550 “junior friends,” which is a Ukrainian word for animals.

Land mines could be a big problem for civilians on the left or eastern side of the river, which is held by Russia. This is where the Kremlin’s military has been building up defenses against the just-started Ukrainian counteroffensive for months.

Natalia Humeniuk, a spokesman for Ukraine’s southern military command, said Thursday on Ukrainian TV that they put minefields there because they were afraid the Defense Forces would get across the Dnipro River. She said that the mines being washed away has caused the Russian forces to move back between 3 and 9 miles.

Officials from both Ukraine and Russia have told people who are leaving their homes to watch out for mines in the floods.

But while the world will be watching what happens on the ground, Newton warned that mines will make it hard for Ukraine to move forward after the war is over.

“People need to realize that you can’t talk about rebuilding a country until you deal with the problem of land mines first,” he said.

Erik Tollefsen, who is in charge of the International Committee of the Red Cross’s Weapon Contamination Unit, said the same thing.

“We used to know where the risks were. Now we don’t know. We only know that they are downriver,” he told Reuters on Thursday.

Tollefsen said that land mines from the Second World War that were found in Denmark in 2015 were still alive and working. This makes it hard to say how long the threat might last.

Greg Crowther, head of programs at the British NGO Mines Advisory Group, also talked about the long-term effects of mine subsidence and warned that unexploded bombs could end up in homes and public places.

“Some of them could be broken and unsafe, buried under mud or flood debris, or stuck in homes, public areas, or gardens,” he said in a statement.

“Some of them may be buried so deeply that it will be very hard to find them in the future, but they could still be dangerous if, for example, they are found while rebuilding after a war.”

Aside from the danger of mines, the collapse of the dam also presents health risks.

In an online briefing on Thursday, Zelenskyy said that the dam break had cut off water sources and flooded warehouses with dangerous chemicals like fertilizer and what he called “anthrax burial” sites in Russian-controlled territory. He also said that sewage lines and places where animals were buried were flooded.

He also said that all of this is going to the Black Sea, which is a key waterway for foreign trade and food supplies.

Videos posted to social media by Ukrainian lawmakers and the military show debris, like a roof, washing up on the shore of Odesa, a port city on the Black Sea.

“Pollution and poison from the flooded area quickly get into the groundwater, poison the rivers, and then get into the Black Sea basin,” Zelenskyy said.

“This means that nature isn’t being destroyed’ somewhere out there’ because everything in the world is very connected.”