A huge Soviet-era dam on the Dnipro River that separates Russian and Ukrainian forces in southern Ukraine was breached on Tuesday, unleashing floodwaters across the war zone.
Ukraine said Russia had destroyed it, while Russia said Ukraine sabotaged it to cut off water supplies to Crimea and distract attention from a "faltering" counter-offensive.
What is the dam, what happened - and what do we not know?
The Kakhovka dam
The dam, part of the Kakhovka hydroelectric power plant, is 30 meters (98 feet) tall and 3.2 km (2 miles) long. Construction was started under Soviet leader Josef Stalin and finished under Nikita Khrushchev.
The dam bridged the Dnipro river, which forms the front line between Russian and Ukrainian forces in the south of Ukraine.
Creation of the 2,155 sq km (832 sq mile) Kakhovka reservoir in Soviet times forced around 37,000 people to be moved from their homes.
The reservoir holds 18 cubic kilometers (4.3 cubic miles) of water - a volume roughly equal to the Great Salt Lake in the US state of Utah.
The reservoir also supplies water to the Crimean peninsula, which Russia annexed in 2014, and to the Zaporizhzhia nuclear plant, which is also under Russian control.
Ukraine, which commented first, said Russia was responsible.
Ukrainian President Volodymyr Zelenskiy accused Russian forces of blowing up the Kakhovka Hydroelectric Power Station from inside the facility, and said Russia must be held to account for a "terrorist attack".
"At 02:50, Russian terrorists carried out an internal detonation of the structures of the Kakhovskaya HPP. About 80 settlements are in the zone of flooding," Zelenskiy said after an emergency meeting of senior officials.
A Ukrainian military spokesperson said Russia's aim was to prevent Ukrainian troops crossing the Dnipro River to attack Russian occupying forces.
Russia said Ukraine sabotaged the dam to cut off water supplies to Crimea and to distract attention from its faltering counteroffensive.
"We can state unequivocally that we are talking about deliberate sabotage by the Ukrainian side," Kremlin Spokesman Peskov told reporters.
Earlier some Russian-installed officials said no attack had taken place. Vladimir Rogov, a Russian installed official in Zaporizhzhia, said the dam collapsed due to earlier damage and the pressure of the water. Russia's state news agency TASS carried a report to the same effect.
What is the human impact?
With water levels surging higher, many thousands of people are likely to be affected. Evacuations of civilians began on both sides of the front line.
Some 22,000 people living across 14 settlements in Ukraine's southern Kherson region are at risk of flooding, Russian installed officials said. They told people to be ready to evacuate.
Ukrainian Prime Minister Denys Shmyhal said that up to 80 settlements were at risk of flooding.
The destruction of the dam risks lowering the water level of the Soviet-era North Crimean Canal, which has traditionally supplied Crimea with 85% of its water needs.
Most of that water is used for agriculture, some for the Black Sea peninsula's industries, and around one fifth for drinking water and other public needs.
The Zaporizhzhia Nuclear Power Plant, Europe's largest, gets its cooling water from the reservoir. It is located on the southern side, now under Russian control.
"Our current assessment is that there is no immediate risk to the safety of the plant," International Atomic Energy Agency chief Rafael Grossi said.
He said it was essential that a cooling pond be left intact as it supplied enough water for the cooling of the shut-down reactors.
"Nothing must be done to potentially undermine its integrity," Grossi said.