A huge dam in the Russian-controlled area of southern Ukraine has been destroyed, unleashing a flood of water.
Ukraine's military has accused Russia of blowing up the dam, while Russian officials have blamed the Ukrainians.
People are currently being evacuated from communities in the surrounding areas, with fears that any flooding could be catastrophic.
Here's what we know so far.
Where is the dam?
The Kakhovka Hydroelectric Power Plant is in the city of Nova Kakhovka in Ukraine's Kherson region, which is currently under Russian occupation.
It was built in the Soviet era and is one of six dams that sits along the Dnipro river, which stretches all the way from the very north of the country into the sea in the south.
It's huge and holds water equal to the Great Salt Lake in the US state of Utah, according to Reuters.
Images circulating on social media this morning show a massive breach in the dam, with water already unleashed across the war zone and flooding downstream in the direction of Kherson.
It is not yet clear how bad the flooding is or will be. But depending on how big the breach is, flooding could be devastating.
The authorities in Kherson, less than 50 miles downstream, have warned residents in low lying parts of the city to evacuate as quickly as possible and seek shelter on higher ground.
The head of the Kherson region, Oleksandr Prokudin, told Ukrainian TV this morning that eight villages had been fully or completely flooded already, with more expected to be flooded.
He said the authorities were evacuating residents by bus and train and 16,000 people were in what he called a "critical zone".
Why has it been attacked?
It's not yet clear what caused the breach in the dam, but Ukraine's military has accused Russia of deliberately blowing it up. This seems plausible, as Moscow may have feared that Ukrainian forces would use the road over the dam to get troops across the river into Russian-held territory, as part of a counter-offensive.
Meanwhile, Russian-installed officials have blamed Ukraine for striking the dam, but they say only the plant's upper was destroyed by shelling - not the dam itself.
Neither Ukraine or Russia's claims have been verified by the BBC
The dam is very important and serves a number of purposes.
It holds back a vast reservoir that supplies water for a host of communities upstream, which means it could affect people's supplies there.
It also provides cooling water to the nuclear power station at Zaporizhzhia, around 100 miles upstream, which is under Russian control and relies on the reservoir.
The International Atomic Energy Agency (IAEA) said there was no immediate nuclear safety risk but it was monitoring the situation.
But as well as that, the dam is a vital channel carrying water from the Dnipro to Russian-occupied Crimea, meaning water supplies there are likely to be affected.
After Russia annexed Crimea in 2014, Ukraine blocked a channel carrying water from Nova Kakhovka, triggering a water crisis on the peninsula.
Russian forces reopened the channel soon after last year's full-scale invasion. But without the dam, dropping water levels could once again jeopardise the flow of water along the channel.
Russia has previously carried out several attacks on dams throughout Ukraine since the invasion, causing widespread flooding and disrupting power supplies.