A suspected underwater sabotage attack off Europe's shores left politicians, security officials and scientists looking for answers on Wednesday to the mystery of what happened to two majority Russian-controlled pipelines leaking gas into the Baltic Sea.
What do seismologists know?
Underwater blasts appear to have rattled the Baltic seabed shortly before the pipeline leaks were discovered, adding to the suspicion of sabotage.
Denmark’s geological survey said they appeared to be man-made and matched the locations where leaks were reported on the Nord Stream pipelines.
“The signals from the tremors do not resemble earthquakes,” it said. "The signals are instead comparable to the signals seen in explosions."
The first had a magnitude of 2.3, equivalent to a small earthquake that might occasionally be felt on land, and was picked up by a number of measuring stations.
Bjorn Lund, from the Swedish National Seismic Network, told broadcaster SVT that there was “no doubt that it was a blast or an explosion”.
German research centre GFZ also detected two spikes but would not be drawn on the nature of the tremors.
What happened to the pipelines?
Two leaks were reported on Nord Stream 1 and one on Nord Stream 2, all in Danish and Swedish waters.
The two parallel pipelines lost pressure and started leaking gas after suffering what was presumed to be substantial damage.
Images taken from Danish military aircraft showed a turbulent Baltic surface near where Nord Stream 2 was leaking. Ships were told to steer clear for fear of losing buoyancy in the water.
There were also environmental fears, especially with methane — the main component of natural gas and a potent greenhouse gas — leaking into the atmosphere from the broken pipes.
Sascha Boden, a climate and energy expert in Germany, estimated that the leak would be equivalent to 15 million tonnes of carbon dioxide entering the atmosphere, though some of the methane might dissolve in the water.
The owners of Nord Stream 1, in which Russian exporter Gazprom holds a majority stake, said it was not possible to say how long repairs would take. Gazprom is the sole shareholder in Nord Stream 2.
How have politicians reacted?
Nato and the European Union described the leaks as intentional. EU foreign policy chief Josep Borrell said any deliberate attack on energy infrastructure would be “met with a robust and united response”.
Denmark, among the first to say it suspected sabotage, sent naval vessels and helicopters to the site of the Nord Stream 2 leak and set up an exclusion zone within five nautical miles (9.3 kilometres) of the source.
Sweden said the leaks were caused by detonations and that it could not rule out “any cause, actor or motive” behind the suspected sabotage. The US and Britain said they were awaiting further inquiries.
While diplomats generally held back from assigning blame to any country, there was much speculation among politicians and commentators that Russia might be behind the leaks.
Why would Russia attack its own pipelines?
A theory doing the rounds in Germany and Poland is that the leaks were a warning shot by Russia, designed to spook Europe with the prospect that other, more critical energy pipelines could suffer the same fate.
Metin Hakverdi, an MP in Chancellor Olaf Scholz’s party, suggested it was an “act of deterrence” by Russia, while opposition MP Roderich Kiesewetter, a former army officer, highlighted Russia’s record of shadowy operations.
“It is likely that Russia is trying on the one hand to sow unease in the European population, and on the other hand to signal on a state level the threat of an attack on critical infrastructure,” Mr Kiesewetter told German radio.
There were raised eyebrows in Poland at the timing of the apparent sabotage just as a new trans-Baltic route was opened between Poland, Denmark and Norway, designed to cut Russia out of the European gas market.
“We see clearly that it’s an act of sabotage, related to the next step of escalation in the situation in Ukraine,” said Polish Prime Minister Mateusz Morawiecki.
With the EU apparently resolved that “the era of Russian fossil fuels in Europe will come to an end”, as European Commission President Ursula von der Leyen has often said, the pipelines are of less use to Russia than they once were.
Russia stopped delivering gas via Nord Stream 1 last month, while Nord Stream 2 has lain idle since Germany suspended the project in February.
For its part, the Kremlin on Tuesday said accusations against Russia were “predictable and predictably stupid”.