Here are the key theories on what caused Ukraine’s catastrophic dam collapse

By Ivana Kottasová and Gianluca Mezzofiore, CNN

(CNN) — The collapse of the Nova Kakhovka dam in southern Ukraine is one of the biggest industrial and ecological disasters in Europe for decades. The catastrophe has destroyed entire villages, flooded farmland, deprived tens of thousands of people of power and clean water, and caused massive environmental damage.

It’s still impossible to say whether the dam collapsed because it was deliberately targeted or if the breach could have been caused by structural failure. The dam and hydroelectric power plant are under Russian control and therefore inaccessible to independent investigators, leaving experts around the world trying to piece together what happened based on limited visual evidence.

Several Western officials have blamed Russia for the disaster, either directly accusing Moscow of targeting the dam or saying that Russia is responsible simply because it is the aggressor in the war on Ukraine.

United Nations Secretary-General António Guterres described the destruction as “another devastating consequence of the Russian invasion of Ukraine,” but added that the UN doesn’t have access to information to independently verify the cause.

A NATO military official told CNN that, while it will take some time before they know for certain who was responsible for the destruction of the Nova Kakhovka dam in Ukraine, they believe Russia was likely behind it. The official added that Russia stood the most to gain by the move, which could potentially slow down an anticipated Ukrainian counteroffensive, if it were to take place in that part of the country.

A number of civil engineering experts have suggested that an explosion inside the structure is the most likely cause of the dam breach, although it’s not the only possible explanation.

Here are the three main theories on what caused the collapse – and what experts and officials say about each:

Did Russia do it?

Ukrainian President Volodymyr Zelensky, his government and the country’s military were quick to blame Moscow for the disaster. They said Russian forces blew up the reservoir from inside, with Zelensky quoting a report by Ukrainian intelligence last year that claimed occupying troops had mined the dam.

The Ukrainians point out that the facility has been under Russian control for the past year, making it easy for Russian forces to plant explosives.

Social media posts indicate that people in the area heard the sound of explosions around the time the dam was thought to have been damaged.

The wider timing of the incident is not insignificant. While Moscow and Kyiv have previously accused each other of plotting to blow up the Soviet-era dam, this collapse coincided with Ukrainian forces gearing up for their widely expected summer counter-offensive.

The dam spans the Dnipro River, a major waterway that has become a front line in the conflict and the scene of heavy fighting in this part of southern Ukraine. The city of Kherson, which sits on the west bank of the Dnipro river, was liberated by the Ukrainian military in November after eight months of Russian occupation. But much of the east bank of the river south of the Nova Kakhovka dam remains under Russian control.

Ukraine’s forces have increasingly taken the battle to Russia’s entrenched front lines in the south and east, and Kyiv has accused Russia of blowing up the dam “in panic.”

Mykhailo Podolyak, a senior aide to Zelensky, said “the terrorists’ goal is obvious – to create obstacles for the offensive actions of the Ukrainian Armed Forces.”

“This once again confirms that the Kremlin is not thinking strategically, but rather in terms of short-term situational advantages. But the consequences are already catastrophic,” he told CNN.

The damage is also affecting the area north of the reservoir, where water levels are falling. The collapse has left 94% of irrigation systems in Kherson, 74% in Zaporizhzhia and 30% in Dnipro regions “without a source of water,” according to the Ukrainian Agricultural Ministry.

The Zaporizhzhia nuclear power plant also lies upstream from the destroyed dam. The reservoir supplies cooling water to the plant, Europe’s largest nuclear power station, and is crucial for its safety. The plant is under Russian control, which has been a major source of anxiety for the Ukrainians, still scared by the 1986 Chernobyl nuclear disaster.

Could it have been a missile attack by Ukraine?

Russia has denied any involvement in the disaster and in turn accused Ukraine of destroying the dam, without providing evidence.

Kremlin spokesperson Dmitry Peskov claimed the attack was “planned and carried out by order received from Kyiv, from the Kyiv regime,” aiming to “deprive Crimea of water” and to distract from the battlefield. Ukraine has denied the accusations.

The reservoir supplies water to large swaths of southern Ukraine, including to the Crimean peninsula which Russia illegally annexed in 2014.

Crimea has experienced water issues ever since Ukraine cut its supply shortly after the annexation. Russian forces captured the North Crimea Canal – which is fed by the Kakhovka reservoir – and began restoring the water supply in the first days of their invasion in 2022.

While the flooding will likely affect any counteroffensive by Ukraine, it’s also impacting Russian forces. Some of the areas worst hit by the disaster are under Russia’s control and have in the past served as staging grounds for Moscow’s military.

There are also suggestions that the dam collapse took at least some Russian forces by surprise.

An officer in Ukraine’s armed forces told CNN that his men witnessed Russian soldiers being swept up in flood waters and fleeing the east bank of the Dnipro River. Capt. Andrei Pidlisnyi told CNN in a telephone interview that when the dam burst in the early hours of Tuesday morning, “no one on the Russian side was able to get away. All the regiments the Russians had on that side were flooded.” CNN cannot independently verify his account.

Russia has accused Ukraine of launching “mass artillery attacks” on the dam, but some experts question whether it would even be possible to cause destruction on this scale from the outside.

Several experts said an internal explosion was a more likely explanation.

“Shelling by Ukraine is highly unlikely as it would need to get massive explosives close to the foundations,” Chris Binnie, a visiting professor at University of Exeter and the chair of Tidal Engineering and Environmental Services, told the UK Science Media Centre.

Craig Goff, the technical director and lead of the Dams and Reservoirs team at HR Wallingford, a civil engineering and environmental hydraulics consultancy, said inflicting enough damage on the dam would require a very precise strike.

“Back in the Second World War, there were the [Royal Air Force] Dambusters attacks on German dams and they had to spend a lot of time working out exactly where to place explosives on the dam in order to cause enough damage to cause it to breach,” he told CNN.

“It wasn’t a simple thing. You had to get the explosives right down on the upstream side of the dam at a deep depth. If it was just the top off the dam then it would probably still survive. You’d lose a bit of water but it would survive,” Goff said.

Structural failure?

The Nova Kakhovka dam – the largest reservoir in Ukraine in terms of volume – is also the furthest downstream of a cascade of six Soviet-era dams on the Dnipro River. The fact that the facility has been operating for many decades has prompted speculation around a possible technical failure.

“The section of dam that we’re looking at is a concrete gravity dam, 35 meters high and 85 meters long (115 feet high and 279 feet long). This is a very common type of dam all around the world. They’ve been built for hundreds of years and if they were designed and built well and are maintained adequately, then the chance of a failure is very, very low. It would be extremely unusual for this type of dam to fail with no warning,” Goff said.

However it is unclear how well the dam has been maintained under Russian occupation. The surrounding area has been one of the most heavily contested regions since Russia launched its full-scale invasion of Ukraine in February 2022 and the dam has sustained some prior damage.

Sections of the northern part of the dam and some sluice gates were also impacted in an explosion in November as the Russian military retreated from the west bank of the Dnipro and Kherson was liberated by Ukrainian troops.

A CNN analysis of satellite imagery from Maxar shows the road above the dam was damaged just days before the structural collapse. The satellite images show the bridge was intact on May 28 but imagery from June 5 shows a section of the same bridge missing. Analysis of lower resolution satellite imagery suggests the loss of the bridge section took place between June 1 and 2.

Meanwhile, data shows water levels in the reservoir behind the dam were at record highs last month, according to the Hydroweb information service.

“The images I have seen show two breaches, either side of a structure. Were the breach to be caused by excess upstream water level there would only be one. Thus natural causes are highly unlikely,” Binnie said.

“The design of the dam will take into account these very high water levels, even extreme, biblical type flooding and there will be spillways to allow the water to go over. So again, the dam shouldn’t fail just because of high water levels,” Goff added.

NORSAR, an independent Norwegian foundation for the detection of earthquakes and nuclear explosions, spotted an explosion at the dam around the time of the first reports of its collapse on June 6, the foundation’s CEO Anne Lycke told CNN.

“We saw this news that the dam burst,” she said in a phone interview. “And then I thought, ‘Let’s go into the data and see if it was an explosion or just an incremental downgrading of the dam that made it burst.’ And then we saw in the data that there had been an explosion close to the dam or at the dam.”

Social media posts indicate that people in the area heard the sound of explosions around 2:18 to 2:20 a.m. local time.

A video of the scene, geolocated by CNN, shows a small explosion at the dam, which appears to be already breached, supposedly at 2:46 a.m. The seismic signals picked up by NORSAR show an explosion, recorded at 2:54am local time, was of magnitude close to 2 – significant but not big, according to Lycke. It remains uncertain whether the event NORSAR detected was the same that caused the dam to collapse. However, Lycke said it was clear the explosion that NORSAR detected was a man-made event and not an earthquake.

Experts are also considering whether a failure inside the power plant could have caused the collapse. Goff pointed to the 2009 explosion at the Sayano-Shushenskaya station, the largest hydroelectric plant in Russia. “In that particular case, there was a problem with one of the turbines. It vibrated and eventually the turbine exploded. And that killed people inside the power house, but it didn’t affect the dam on that in that instance, because of the way the dam was built,” he said.

“It is possible that if the hydropower station was at a critical point inside the dam and that something bad happened in that power house that possibly could have caused an explosion inside that would damage the dam,” Goff said. He added, however, that it would be “extremely unlikely” for such an accident to happen without advance warning.

“You would know how to operate the dam safely and you would know that the turbines shouldn’t be vibrating that much… so if it was being looked after properly, you can probably rule that one out,” he said.

But as the plant had been under Russian control for over a year, no one can be sure what was happening inside during that time, and it’s far from certain if those who operated it knew what they were doing.

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CNN’s Vasco Cotovio, Allegra Goodwin, Sebastian Shukla, Sam Kiley, Natasha Bertrand, Alex Marquardt, Jim Sciutto and Jennifer Hansler contributed reporting.