In the early hours of September 26 last year, seismologists detected something in the Baltic Sea where the $16 billion Nord Stream gas pipelines connect Russia and Europe.
Explosions had torn apart the pipes, and it became clear that the highly controversial infrastructure, meant to be a major supplier of energy to Europe, had been sabotaged.
Ukraine immediately pointed the finger at Russia, accusing it of wanting to "destabilise" Europe, while Moscow accused the United States.
Earlier this month, a New York Times story suggested that a non-state actor supportive of Ukraine was to blame for the explosion.
And since then, German media have reported that traces of explosives were found on a mysterious yacht seized by local authorities.
So what exactly are the theories as to who was behind the incident, and how credible are they?
What happened to the Nord Stream pipelines?
The explosions were recorded near the Danish island of Bornholm in the Baltic Sea.
Seismic stations in Sweden, Norway and Finland detected two blasts — one south-east of the island and another north-east of the island.
The blasts happened in international waters but within the economic zones of Denmark and Sweden.
The explosions released gas that bubbled on the surface of the Baltic Sea for days, with the largest leak causing a sea surface disturbance more than 1 kilometre in diameter.
Danish officials said the leak amounted to the equivalent of one-third of Denmark's total annual greenhouse gas emissions being released into the atmosphere.
Traces of explosives were found at the site of the explosion, according to Swedish prosecutors.
"Analysis carried out shows traces of explosives on several of the foreign objects that were found," Mats Ljungqvist from the Swedish Prosecution Authority said.
Sweden's then-prime minister Magdalena Andersson said the blasts were a "deliberate act" that was likely "sabotage".
Theory 1: A Russian 'terrorist attack'
Soon after the explosion, Ukrainian presidential adviser Mykhaylo Podolyak said, without providing evidence, that the blast was a "terrorist attack" planned by Russia to destabilise Europe.
"Russia wants to destabilise economic situation in Europe and cause pre-winter panic. The best response and security investment — tanks for Ukraine," he said in a tweet.
The Kremlin rejected the accusations, with spokesman Dmitry Peskov saying it was "very important to find those who are behind the explosion".
Claudio Bozzi from Deakin University Law School, an expert in maritime law, told the ABC the Baltic Sea is heavily surveilled and very busy, making any attempt to detonate an underwater bomb difficult.
Dr Bozzi said given Russia's commitment to resources in Ukraine, and the fact that it isn't in Russia's interest to destroy the pipelines, made it unlikely that Moscow was the culprit.
"There was very little in Russia's interest, except possibly an insurance claim, because Nord Stream 2 was never operational," Dr Bozzi said.
Associate Professor of Central Asian Studies at the Australian National University, Kirill Nourzhanov, told the ABC that Russia wouldn't need to blow up the pipelines to Europe because it already had the power to shut off supply.
Theory 2: The US ordered the explosion
Russia's UN ambassador Vassily Nebenzia told the UN Security Council last year that Moscow wanted UN secretary-general António Guterres to establish an independent international investigation of the explosions.
Mr Nebenzia said Moscow had "no trust" in the separate investigations being carried out by Denmark, Sweden and Germany because Russia was not allowed to join the probes.
In a proposed resolution, Russia alleged that the US was behind the explosion and said the blast "occurred after the repeated threats to the Nord Stream by the leadership of the United States".
US diplomat John Kelley denied US involvement in remarks at the UN, saying: "Accusations that the United States was involved in this act of sabotage are completely false. The United States was not involved in any way."
In February, investigative journalist Seymour Hersh published a Substack article titled "How America Took Out The Nord Stream Pipeline:", alleging the US carried out the blast with the approval of President Joe Biden.
Hersh famously exposed a massacre in the Vietnam War, where US soldiers slaughtered over 500 civilians in the South Vietnamese village of My Lai.
In his article, he claimed Navy divers planted remotely triggered explosive devices on the pipelines under the guise of NATO exercise BALTOPS, before the blast was authorised.
He said the US was assisted by the Norwegian Navy.
"A Norwegian Navy P8 surveillance plane made a seemingly routine flight and dropped a sonar buoy," Hersh said.
"The signal spread underwater, initially to Nord Stream 2 and then on to Nord Stream 1. A few hours later, the high-powered C4 explosives were triggered and three of the four pipelines were put out of commission."
His article referenced comments made by Mr Biden before the invasion of Ukraine began that the US would "bring an end" to Nord Stream 2 if Russia went ahead with the attack.
A spokesperson for the White House National Security Council dismissed the article as "utterly false and complete fiction".
Theory 3: A 'pro-Ukraine group' did it
Earlier this month, the New York Times reported that US officials had intelligence suggesting that a "pro-Ukraine group" carried out the blast.
Officials said there was still a lot they did not know about the alleged perpetrators, but that they were opponents of Russian President Vladimir Putin and likely Russian or Ukrainian citizens, according to the New York Times story.
There was no evidence that Ukrainian President Volodymyr Zelenskyy or other Ukrainian government officials were behind the attacks, it said.
The report said that the explosives which tore the pipes apart were most likely planted with the help of experienced divers, some of who may have previously received specialised government training.
Mr Podolyak, the Ukrainian presidential adviser, said that Kyiv was "absolutely not involved" in the blasts and had no information about what happened.
Dr Nourzhanov said if a pro-Ukrainian group was involved, even if not connected to the Ukrainian government, it would be damaging to Ukraine's reputation.
"Because the image of Ukraine as a nice, democratic country fighting an evil aggressor will be tarnished," he said.
"The Ukrainian trace would inevitably lead to some kind of cooling down in the relationship between Ukraine and the EU."
Fake passports and a yacht with traces of explosives
German media recently reported that a team of divers operating a chartered yacht sailed from its base in Rostock, Germany, to near the Danish island of Christiansø around the time of the incident.
Broadcaster ARD and the newspaper Zeit said, without citing sources, that German investigators were able to identify the boat.
A group of five men and one woman, using forged passports, rented the yacht from a Poland-based company owned by Ukrainian citizens, the German media outlets reported.
The nationality of the sailors was unknown.
Investigators found traces of explosives on the yacht, which the group took on September 6 last year, according to ARD and Zeit.
Experts have questioned whether the 15-metre boat could have held enough explosives needed to inflict damage on the pipelines.
Dr Bozzi said bombing the Nord Stream pipelines would have been a "very complex operation" and that it was unlikely a breakaway organisation or non-state actor would be able to undertake such a task.
"It's hard to operate on the surface of the water at the best of times, let alone under the ocean," Dr Bozzi said.
"To have the technology, the means to do it and to have not just the means to undertake an operation but all that's required to survey, get the intelligence and the access."
How credible are all these theories?
Dr Nourzhanov said the theories pinning blame on a pro-Ukrainian group and the US had flaws because the stories rely on anonymous sources.
"Hersh claims that this [his] source is in the know within the US intelligence community, but again there is no disclosure or triangulation of information.
"Everything is anonymous."
Dr Bozzi said there was no clear, direct or even indirect information to indicate who was involved in the Nord Stream pipeline explosion or how it was carried out.
"I believe there are a lot of gaps in what is known. I think those gaps need to be filled. I think the investigations are releasing fairly guarded … information," Dr Bozzi said.
He said investigators have steered clear of releasing information about the incident as they try and work out exactly what happened.
"No questions are going to be answered by speculation, and we may never have enough information to ultimately determine what happened and who did it," he said.
"It might be a very complex set of answers that we come up with, none of which are particularly conclusive."
ABC/Reuters
