Torrential rain, loud explosions and burst dams.
With thousands feared dead, Sky News looks at what could have caused the devastating flood that wiped out a quarter of the eastern city of Derna.
Satellite images show the Wadi Derna river, which runs from the mountains through the city and into the sea, has been dry for most of the year, though it floods semi-regularly when it rains.
But on Sunday night, Storm Daniel pounded the coast and residents of Derna reported hearing loud explosions before they realised dams outside the city had collapsed, unleashing flash floods down Wadi Derna that eventually crashed into the city.
Some 414mm of rain fell on the nearby town of Bayda, according to a local weather organisation, which the Sky News weather team described as a “huge amount of rainfall” that likely quickly ran off rather than soaked into the hard dry ground.
The first dam is about 12km upstream from the city where two river valleys converge, and footage has shown it has been completely destroyed.
After breaching the first dam, the floodwater would have continued downstream along the canyon towards the second, which sits on the southern edge of the city.
More on Data And Forensics
Debris was left strewn throughout the city
Downstream of the second dam, a wide torrent of water cascaded through the city centre, smashing into buildings on either side.
Multi-storey apartment blocks that were well back from the river had their facades ripped away and concrete floors had collapsed, while cars lifted by the flood were left stacked on top of each other.
Estimates suggest 2,000 may have died in Derna alone
Libya asks for international help after storm
Cars lifted by floodwaters ended up piled on top of each other
Researcher warned repeated flooding threatened city
In a research paper published last year, hydrologist Abdelwanees A R Ashoor of Libya’s Omar al Mukhtar University warned repeated flooding of the seasonal riverbed, or wadi, was a threat to Derna, citing five floods since 1942.
His paper called for immediate steps to ensure regular maintenance of the dams and added: “If a huge flood happens the result will be catastrophic for the people of the wadi and the city.”
Floodwaters left swathes of mud and wreckage behind
The storm also hit other areas, including the town of Bayda, where around 50 people were reported dead.
Hundreds of families were also displaced and took shelter in schools and other government buildings in Benghazi and elsewhere in eastern Libya.
The Data and Forensics team is a multi-skilled unit dedicated to providing transparent journalism from Sky News. We gather, analyse and visualise data to tell data-driven stories. We combine traditional reporting skills with advanced analysis of satellite images, social media and other open source information. Through multimedia storytelling we aim to better explain the world while also showing how our journalism is done.