Ukraine is now living through its most difficult winter in recent memory.
With January temperatures plummeting below -15C, Russia has been attacking energy infrastructure, leaving about a million Ukrainians without heating.
The capital, Kyiv, is the main target of such attacks. Following the latest Russian bombardment overnight into 24 January, almost 6,000 apartment blocks were left without heating, according to mayor Vitaly Klitschko.
This is the third such Russian attack targeting Kyiv's heating infrastructure in little more than two weeks, following strikes on 9 and 20 January that also left hundreds of thousands freezing in their flats.
Living in Kyiv is a bit of a gamble these days, one resident of the Ukrainian capital, Rita, told the BBC.
If you have heating and gas, there is no electricity and water. If you have electricity and water, there is no heating. Coming home is like playing a guessing game every day - will I be able to shower or have hot tea, or neither? And of course missiles and drones come on top of all that.
She says she has to go to bed wearing a hat and several layers of clothing.
What is making things much worse for Ukraine and easier for Russia is the widespread prevalence of apartment blocks that rely on communal central heating - where water is heated up elsewhere and then pumped into their radiators.
Heating plants in Ukraine are huge, and many thousands of people are affected when they are targeted by Russian forces. Ukraine says that all such power plants have now been hit.
Such attacks also disrupt electricity supplies, but while a generator or battery pack might help in this situation, heating is less straightforward - especially when there is also no electricity to power your heater.
Kyivteploenergo, the monopoly supplying heating and hot water in the Ukrainian capital, told the BBC that the absolute majority of houses in Kyiv rely on its services, but they could not share the exact number for security reasons.
In Zaporizhzhia, a frontline city home to 750,000 people, almost three-quarters of residents rely on central heating, according to Maksym Rohalsky, the head of the local association of apartment block dwellers.
Before Russia's full-scale invasion of 2022, about 11 million households in Ukraine relied on central heating, compared to seven million autonomously heated households, Ukrainian energy expert Yuriy Korolchuk said.
Cities across the Soviet Union, including in Ukraine, were the focus of huge construction programmes launched in the 1950s to mass produce cheap housing. The landscapes of cities in the former USSR are dominated by ubiquitous nine-storey residential buildings made from pre-fabricated concrete panels, known as panelki, or smaller five-storey blocks of flats known as khrushchevki, after Soviet leader Nikita Khrushchev, who oversaw their construction.
Heating to such houses is supplied by large plants known as TETs - an acronym that stands for heat and electricity centrals in Ukrainian as they generate electricity as well as heat.
Ukraine inherited the Soviet heating system and it hasn't changed anything; it stays predominantly centralized, Korolchuk told the BBC.
He added, These heating plants were not designed to be attacked with missiles or drones, that's why these vulnerabilities came to the fore during the war. This is a new tactic used by Russia, aiming to cause more hardship during winter months.
According to him, During the previous winters, there were no such strikes against the heating system. They happened only occasionally, and they didn't directly target heating plants. As negotiations to end the war continue, this tactic appears to be used as a form of pressure on Ukraine.
Large centralised installations bring about efficiencies of scale, but when targeted, the consequences can be devastating for hundreds of thousands of people. The Ukrainian government is conscious of this vulnerability and is planning to reduce it by mandating individual heating points at apartment blocks, but changing decades of Soviet urban planning will not be quick or easy.
