The Nord Stream Explosion Remains an Unsolved Mystery Gripping Europe

The journey took less than a second. But the information sat unexamined on the servers of the Geological Survey of Denmark and Greenland (GEUS) for more than 24 hours, until Nicolai Rinds, a tall seismologist with a shaved head, sat down at his desk with a cup of coffee and booted up software to examine the previous day’s seismic activity. Denmark is hardly noted for its volcanoes or earthquakes, so the jagged spikes from the Bornholm seismograph struck him as odd.

In the staff room, the morning chatter quickly turned to media reports of gas bubbling up from the Baltic seabed and turning the surface into a gurgling cauldron. “If there’s a big break from a huge pipeline, can we see it?” Tine Larsen, a colleague of Rinds, remembers asking. When Rinds got back to his desk, he reexamined the Bornholm readouts against other seismographs, hoping to clarify his initial disquiet and triangulate a location. The pressure waves recorded by two other devices—one in Denmark, one in Sweden—showed the shock had emanated from a single geographic point rather than a wider area, as typically seen in an earthquake. He beckoned colleagues over to validate his findings. “None of us had any doubt,” Larsen recalls. “Not an earthquake.”

The seismologists alerted their agency’s director, who in turn informed his boss, Denmark’s climate minister: There’d been some sort of underwater explosion. Other gas bubbles were spotted farther to the north, and when the team examined the seismic readings, they saw similar data. Each location appeared to correspond to a point on the route of the Nord Stream 1 and 2 pipeline systems built to bring Russian gas to western Europe. Although the newer Nord Stream 2 hadn’t yet opened, Nord Stream 1 had at its peak supplied almost 60 billion cubic meters of gas each year, fully two-fifths of Europe’s overall supply. “Having two explosions on gas pipelines within the same day is not likely to be an accident,” Larsen remembers thinking. “Whatever it is, it’s not natural.”

Within hours, leaders of the Danish police, military and intelligence services were huddled at the national security council. Denmark had never actively supported the plan, first floated in the late 1990s, to pump Siberian gas hundreds of miles under the sea from a remote Russian bay to northern Germany. But in 2009, Copenhagen allowed two of those pipelines to pass through a part of the Baltic that was simultaneously considered international waters and an “exclusive economic zone” of Denmark because of its proximity to Bornholm. The seismologists at GEUS had quickly determined that the first blast likely happened in that jurisdictional gray area. Danish and Swedish authorities eventually detected four separate pipeline leaks, all in economic zones belonging to one of the two countries yet still in international waters. According to multiple current and former Danish government and military officials, the locations suggested a precisely planned attack.

At a hastily called United Nations Security Council meeting four days after the blasts, Marc-Antoine Eyl-Mazzega, director of the Energy & Climate Center at the French Institute of International Relations, said any targeting of a multibillion-dollar piece of transnational energy infrastructure would set an “extraordinary precedent.” The explosions, Eyl-Mazzega told the assembled diplomats, indicated an escalation of geopolitical tensions between Russia and NATO, “where energy trade and pipelines have taken center stage—and been weaponized.”

A slew of global leaders had by then determined that the blasts were deliberate and called for the perpetrators to face serious consequences. But in the half-year since, a contentious debate has raged over the question of responsibility. Policymakers and military commanders—particularly in the Baltic region—have come up with a host of theories about the explosions. Multiple countries are investigating, though none has gone public with conclusive results. In the meantime, governments in Europe and beyond are reconsidering their military doctrines and reviewing security options for hard-to-defend infrastructure.

The timing of the attacks seemed anything but coincidental. The same morning the seismologists were poring over the data, Danish Prime Minister Mette Frederiksen was in Poland for the launch ceremony of another conduit, known as the Baltic Pipe. It would carry Norwegian gas via Denmark to Poland’s Baltic coast, bolstering European supplies as the continent works to end its decades-long reliance on Russia’s hydrocarbons. “We have to do all we can to remove energy as a Russian instrument of power,” Frederiksen said. “Together we will defeat Putin.” The Nord Stream blasts visibly demonstrated the vulnerability of such infrastructure.

The twin pipelines of Nord Stream 1, which opened in 2011, had been closed for maintenance in August, six months into Russia’s war in Ukraine. The pipeline’s supplier, Gazprom PJSC, said the upkeep would take longer than normal because of sanctions levied on Russia after the invasion. Only hours before gas flows were set to resume in early September, Group of Seven leaders announced plans to initiate a price cap on Russian oil, and Ursula von der Leyen, the President of the European Commission, suggested a similar move for gas. That prompted Gazprom to announce that Nord Stream 1 shipments would be halted indefinitely.

Nord Stream 2, a pair of pipelines that run almost parallel to Nord Stream 1 while skirting the Danish economic zone by a few miles, was completed in 2021 but never entered service. Although German and Russian leaders had pushed the project, it had faced vehement opposition from the US, Poland and other European allies. In the days leading up to the invasion, Berlin finally pulled its support for the project, with Chancellor Olaf Scholz banning any gas sales via Nord Stream 2 that might financially benefit the Kremlin.

So at the time of the blasts, the two systems constituted little more than a 760-mile stretch of virtually worthless steel and concrete. Nonetheless, they were filled with millions of cubic meters of natural gas, mostly methane, to maintain internal pressure and keep them from collapsing under the weight of the water above; that was the gas seen churning up the Baltic’s surface in the hours after the explosions. A Gazprom official told the UN Security Council that at the time of the explosion the system contained about 800 million cubic meters of gas, a quarter of Denmark’s annual consumption.

Further investigation revealed damage so extensive that any quick resumption of flows seemed highly unlikely, putting underwater physics suddenly in sync with aboveground politics: The attacks drastically cut the potential supplies just as everyone in Europe wanted to stop buying Russian gas. “Nobody really knew what was going on,” recalls Captain Dan Termansen, commander of the Danish navy’s second squadron. “The whole context of Ukraine, of course, was part of the thinking.”

Termansen joined the navy in the 1980s, during the Cold War, when its sailors maintained a permanent state of readiness. With just an hour’s notice day or night, commanders could launch fast-attack vessels to engage with any potential Soviet landing force. “The mission back then was very much focused on the Baltic,” he says, clambering up a steep staircase to the bridge of the , a frigate loaded with antiaircraft guns and missile launchers.

Staring out over the harbor of Korsor, guarded by a red-brick fortress dating from the 12th century, Termansen points out the Storebaelt (“Great Belt”) strait, which serves as a gateway to the Baltic. Under international law, Russian vessels must announce their passage through it on their way to the Atlantic, with submarines required to surface and remain visible until they depart Denmark’s territorial waters. After decades of relative calm, when Danish boats have gone much farther afield to complete missions for NATO, he says, the Baltic is once again top of mind for his country’s navy.

More than 100,000 mines from 20th century wars still litter the Baltic seabed, occasionally turning up in the nets of fishing trawlers, and Denmark employs several dozen divers dedicated to clearing them. Just over 24 hours after the first Nord Stream blast, with Prime Minister Frederiksen calling the incident “as serious as it gets,” a dozen members of that elite group were on their way to examine the ruptured pipes close to Bornholm. The divers, instructed not to discuss their endeavors with even some of their own commanders, began collecting pipeline and seabed samples in the “Bornholm Deep,” a dumping ground for World War I mustard gas containers and thousands of deteriorating, but still dangerous, old mines.

Such work can require long periods in dark waters breathing air from tanks filled with heliox, a mixture of helium and oxygen—and no nitrogen, due to its potentially narcotic effects. Over beers in a Copenhagen bar, one of Denmark’s most experienced navy divers says finding the pipelines without precise coordinates or tracking technology, then transporting and placing the explosives, would’ve challenged diver-saboteurs. The charges, which likely weighed hundreds of pounds, had been placed against the underside of the pipes, with the velocity of the explosions helping avoid a fireball that might have ripped along the pipelines in both directions. The diver estimates the operation, including a safe ascent, would’ve taken an individual diver several hours. He surmises that whoever carried out the attack had access to a fast-moving autonomous submersible vehicle, like the ones employed by sophisticated naval forces. A surface vessel remaining relatively static for hours as it waited for a dive team, he says, would’ve attracted unwanted attention in the moment or been flagged by investigators after the fact.

The Danish navy says the hypothesis is sound, as it would’ve been difficult for divers to spend the time required to set up an attack at such a depth without being detected. The Copenhagen police overseeing the criminal investigation, Denmark’s domestic intelligence agency and its defense and foreign affairs ministries had no further comments.

In neighboring Sweden, the threat represented by the Nord Stream attacks is starting to reshape military strategy. Serving a country with about 2,000 miles of coastline and 230,000 islands, the Swedish navy has long excelled at undersea surveillance. And those skills have been beefed up over the past two decades as Russian submarines have increasingly probed the country’s waters. With highly varied salinity levels, ever-shifting currents and often shallow depths, the Baltic makes covert submarine navigation anything but easy. Sweden’s expertise represents a useful tool for NATO, which the country aims to join after centuries of neutrality, a dramatic turn triggered by the Russian invasion. “In this environment we are experts,” says Brigadier General Patrik Gardesten, tapping a map of Baltic shipping routes on a table in a dark, wood-paneled room on Musko island.

About an hour’s drive south of Stockholm, the island is home to a naval base reminiscent of a villain’s lair from a James Bond movie, with vast subterranean docks linked by a web of tunnels blasted into the rock. Gardesten, a marine officer who was appointed deputy chief of the navy just a month before the Nord Stream blasts, says the incident illustrates the growing risks of what’s often known as hybrid warfare, the targeting of strategic civilian infrastructure. “It has a huge impact on how our citizens feel about their own security and stability in the region,” he says.

With his boss away in the US on the day of the explosions, Gardesten had rushed to the naval operational center for a briefing. Minutes later he picked up an emergency phone linked to an encrypted NATO communications system. A British admiral answered. “If you are daring to call me, I will support you,” Gardesten recalls the admiral telling him. Gardesten’s voice cracks at the memory of NATO counterparts calling to offer assistance. “It was a feeling that you are not alone in this, that you have more or less the whole Western world behind you.”

Sort of. There’s been little public collaboration on investigations of the blasts among the various military powers. Denmark, Germany, Poland and Sweden have launched independent inquiries, as has Russia, whose permanent representative to the UN, Vassily Nebenzia, soon called for “criminal proceedings under the article on acts of international terrorism.” Subsea military capabilities remain a significant source of mutual distrust and suspicion. The Swedish investigation’s lead prosecutor, Mats Ljungqvist, has said his team’s efforts involved information that was “subject to confidentiality directly linked to national security,” so Sweden wouldn’t participate in any joint investigation that might oblige it to share such information. Despite years of cooperating on training missions, Swedish and Danish diving teams stayed away from each other while examining the blast sites, sharing neither equipment nor any evidence they gathered.

The roots of this caution are easy to see. Hundreds of Russian spies have been expelled from European capitals in the past year, and the allies’ concerns about military secrets being passed to Moscow were underlined in December by the arrest of alleged Russian moles inside Germany’s foreign intelligence agency, the BND. In Poland, authorities in January seized three men who claimed to be Spanish nationals close to energy installations, prompting a security review. In a single week last October, Norwegian police arrested seven Russian nationals in four incidents of flying drones over or photographing sensitive defense or energy installations. With Norway’s thousands of miles of pipelines now carrying the majority of Europe’s natural gas, the attack was “a wake-up call,” says Paal Hilde, a former defense ministry official who works at the Norwegian Institute for Defense Studies. “All our infrastructure can be at risk.”

“A lot has changed since the explosions,” says Eerik-Niiles Kross, the former head of Estonian intelligence and now a member of its Parliament. Calling himself a “paranoid Balt,” he says he’s more concerned about a direct military threat like a Russian invasion than a one-off attack on a gas system or electricity grid. But he acknowledges that countries relying on infrastructure such as underwater pipelines remain vulnerable. “Which brings us to the question: Who did it?” he says. “Does it make sense for the Russians to do it? I mean, yes—and no.” He surmises that while some intelligence agencies may have evidence identifying the saboteurs, they could have ample reason not to release it—“mostly if it’s not the Russians.”

Kross was engaging in what’s become one of the most intense geopolitical parlor games in decades. From the nine countries that ring the Baltic, to other European capitals, and across the Atlantic, the identity of the culprits has flummoxed politicians and policy wonks while prompting circular finger-pointing. Western and Ukrainian officials initially suggested Moscow was responsible. The Kremlin called that “predictably stupid and absurd,” then a month later blamed the British. The UK described Moscow’s accusation as “peddling false claims on an epic scale.” Germany’s attorney general in February said Russian involvement “cannot be proven.” Swedish authorities have called the incidents sabotage but, like the Danes, they aren’t publicly saying much more.

In March, the , the and some German media outlets reported that intelligence reviews suggest a link to a “pro-Ukrainian group” or even Ukraine itself. Following one such report, an adviser to President Volodymyr Zelenskiy’s office wrote on Twitter that although he enjoyed collecting “interesting conspiracy theories” about Ukraine’s government, the country “had no connection to the incident in the Baltic Sea.”

Those reports followed a blog post by Pulitzer Prize-winning investigative reporter Seymour Hersh that placed responsibility on the Biden administration, with help from Norway. Hersh, who has faced criticism in recent years for his reporting about events in Pakistan and Syria, said his account of the Nord Stream blasts came from a single source described as having “direct knowledge of operational planning.”

The US ambassador to NATO, Julianne Smith, insists “the United States bears no responsibility for what happened, and any suggestion of that kind is preposterous.” John Sipher, a 28-year veteran of the CIA’s National Clandestine Service who’s now at the Atlantic Council, says no NATO country would get involved in such an operation. “All of our militaries have that ability to do these kinds of things, but doing them in peacetime—I don’t see the big benefit,” he says. As for a US role, he says, “I just can’t imagine: At every level of that chain, people would say, ‘This is the dumbest thing I’ve ever heard. There’s no way we can or should do this.’ ”

Ronald Marks, a former CIA case officer with experience in Russian operations, is increasingly convinced the blasts were carried out by a small group of Ukrainians, perhaps with help from a foreign government. “I’d be willing to bet a mortgage payment on it,” says Marks, currently a visiting professor at George Mason University.

Fiona Hill, a Russia expert who’s served on the US National Security Council, said on a recent podcast that she initially thought Russia had blown up the pipelines, “to basically signal that a lot of other infrastructure could be imperiled.” But she said she’d come to wonder whether the Ukrainians could be responsible—while insisting she’s seen no evidence of this. Samuel Charap, a senior political scientist at Rand Corp. who does research on Russia, is reasonably certain Moscow didn’t blow up pipelines that Russian companies spent billions to build. “If there were even a slice of evidence it was Russia,” he says, “it would have leaked out long ago.”

Across northern Europe, and particularly in Denmark, the Nord Stream explosions and the invasion of Ukraine have refocused military minds while radically altering how politicians and industry leaders think about energy infrastructure. A web of pipelines already crisscrosses Europe and stretches beneath the North Sea and the Mediterranean. As the region seeks to wean itself off hydrocarbons, several countries are constructing vast offshore wind farms linked to the shore by cables that will be extremely hard to fully safeguard against attack.

In his office at the University of Copenhagen’s political science department, behind the forbidding brickwork of a former 19th century hospital, Jens Wenzel Kristoffersen has spent hours examining details of the blasts, working his way through scenarios on his outsize computer monitor. Kristoffersen, a naval officer and academic, posits that intelligence agencies worldwide will have combed through satellite imagery using powerful algorithms, comparing that with ship transponder data in an effort to identify suspicious vessels. He talks about a Russian warship that last June violated Danish territorial waters close to Bornholm, when the island was hosting a gathering to celebrate Denmark’s democracy. One Danish theory is that just such a vessel could’ve installed charges with delayed detonators to destroy the pipelines. (The Russian Embassy issued a statement saying Denmark had presented “no evidence of what happened” regarding any breach of its waters.)

Kristoffersen finds discussions about culpability and capability fascinating but ultimately considers them rather irrelevant; what really matters is his country’s long-term resilience. For Denmark and its neighbors, the conflict in Ukraine has prompted a profound debate about defense spending. With their limited military resources, many smaller countries must project force overseas as members of NATO, guard their own backyard against Russian aggression and, increasingly, secure remote energy infrastructure. “There’s no submarines in Denmark,” Kristoffersen says heatedly, noting the country did away with its costly sub program two decades ago. “How are we able to survey our subsea infrastructure or respond to any kind of aggression coming from the east?”

Denmark is in the midst of debate about defense spending for the next decade, and the Nord Stream strikes have brought underwater drones and mine clearance equipment to the forefront of that discussion. Kristoffersen endorses such investments as necessary protection from sabotage. “There’s no constant surveillance of any of the sub cables, be it communication, be it electricity, be it gas,” he says, rapping the table with his knuckles in frustration. “We’re going from 2.3 gigawatts of wind turbines to 8.3 in 2030, and what are we doing in terms of protecting these? Absolutely, to my knowledge, zero.”

Across the North Sea, the UK is expanding its own offshore wind capacity while also beefing up its ability to protect the turbines and related underwater infrastructure. It’s purchasing a pair of ships designed to maintain offshore installations for oil and gas companies and retrofitting them for military use. The first, the , is scheduled to become operational (with a new name) this summer. Built in Norway, the ship can conduct surveys and inspections as well as launch autonomous submarines and robots from a hidden underwater opening inside its 6,600-ton hull. “It is paramount, at a time when we face Putin’s illegal invasion of Ukraine, that we prioritize capabilities that will protect our critical national infrastructure,” Ben Wallace, the UK defense minister, said in January.

France last year outlined plans for its military to develop underwater drones and robots alongside the country’s defense contractors. NATO and the European Union have set up a joint task force focused on critical infrastructure. In February, Secretary General Jens Stoltenberg announced the formation of a “coordination cell” at NATO’s Brussels headquarters, to be led by a veteran German officer. The group will be charged with improving communications about underwater security among civilians, militaries and private companies across the alliance.

The consequences of not acting to protect such infrastructure are now clear. Gazprom, which has seen its gas exports tumble, said a few days after the September blast that it had started investigating ways to restart the Nord Stream system but could provide no time frame for repairs. It quickly became apparent why. In October a Norwegian company that makes undersea drones explored the area with a remote-controlled vehicle steered by a repurposed Xbox joystick and powered via a 1,000-foot cable. Despite sometimes cloudy water, the drone captured video showing severe deformities in the pipeline’s metal circumference, twisted rebar from the concrete casing and an extensive gash in the seabed where a long section of the pipeline had once run. A few days later a ship dispatched by Gazprom, the , was permitted to investigate in the Swedish economic zone. It reported finding craters 3 meters to 5 meters deep, with sections of the pipe dispersed over an area of at least 250 square meters.

To detect threats and prevent such damage happening again across the Baltic will require more sonars, submarines and mobile and immobile sensors. “It’s such a huge task,” acknowledges Gardesten, the Swedish marine general. He invited Baltic naval chiefs—excluding Russia’s—to a meeting at his Musko island redoubt in March, to discuss underwater defense. “It’s important to show our friends, our partners and our opponents that ‘We are here, we see what you do,’ ” he says. “And we show our will to defend our interests and our territory.”

In the frantic days following the Bornholm blasts, red lines and threats of retaliation were thrown down by various Western leaders. The EU’s chief diplomat, Josep Borrell of Spain, declared: “Any deliberate disruption of European energy infrastructure is utterly unacceptable and will be met with a robust and united response.” More than six months on, if anyone has identified the culprits, that hasn’t been made public. And if there was any sort of response along the lines Borrell suggested, like so much of what happens under the sea, we simply don’t know about it. Perhaps we never will.

(Adds Von der Leyen comment in paragraph below the map. An earlier version of the story corrected the description of the price cap.)

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