Thousands of ships navigate northern Europe's Baltic Sea each day.
Among the world's busiest shipping routes, this vast arm of the Atlantic Ocean has connected the region to global trade for centuries.
In recent years, however, what lies beneath the sea's surface has been more important to governments than what sails above: the myriad of subsea internet cables which power their modern economies.
Last month, to the alarm of governments in close-by Latvia and Sweden, one of those cables mysteriously malfunctioned in what appeared to be an act of sabotage. It left top engineers scrambling to squash the threat of disruption to citizens' lives.
NATO deployed patrol ships and Swedish authorities seized a tanker they believed may have been involved.
On this occasion, the move did not spark a serious crisis. But warfare experts have warned such behaviour is on the rise — and could have impacts around the world, including in Australian waters.
Suspect emerges in Europe's 'shadow war'
Latvia's prime minister, Evika Silina, said publicly the damage to the fibre optic cable was caused by an "external impact".
It mirrors other incidents in the Baltic Sea where data, electricity and gas connections have been targeted — all suspected to be part of what intelligence agencies have called a "hybrid war" being waged against Western Europe.
Gabrielius Landsbergis, who served as Lithuanian foreign minister between 2020 and last year, says suspicion has fallen on one country in particular.
"We know that it has been Russia. It has been publicly proven by cooperation between our intelligence agencies," he tells the ABC.
"It is not a coincidence — Russia is involved in what I call a shadow war activities against against the West."
Lithuania, which, like Sweden, is a NATO member, has seen its own underwater sea links in the Baltic targeted.
Mr Landsbergis says other incidents in the country, such as a seemingly-random fire at an IKEA and a plot to blow up cargo flights over the Atlantic, are also suspected to have been sophisticated operations backed by a foreign actor.
He believes it is part of a coordinated effort by the Kremlin, which alongside the various misinformation campaigns Vladimir Putin's regime is suspected to be behind, is designed to limit Western support for Ukraine.
Donald Trump's return to the White House had raised hopes that a peace deal could be struck between Ukraine and Russia, but talks have yet to materialise. The US president has urged Mr Putin to "make a deal" and threatened further sanctions and tariffs on Russia if Moscow continues to refuse.
It means Russia and its allies could continue to be a mischievous presence for Ukraine's Western allies.
Marion Messmer, a senior researcher at Chatham House, points out that after Sweden and Finland's decision to join NATO in 2022, Russia is the only country bordering the Baltic not part of the military alliance.
"The Baltic Sea has seen a lot of increased activity ever since the Russian invasion of Ukraine in early 2022," she says.
"In the wake of Finland and Sweden making the decision to join NATO, the number of incidents in the Baltic Sea went up pretty rapidly."
[EXTERNAL LINK: Baltic Sea]The grey zone of war
Dr Messmer says Russia's alleged plots have focused on former Soviet states, like Lithuania, Latvia and Estonia, who have moved to separate their countries' network from Moscow's electricity grid following the invasion of Ukraine.
"Damaging that kind of electricity infrastructure sends a big signal that Russia is watching, and Russia isn't too much of a fan of them essentially increasing that independence."
Gorana Grgic, a senior researcher at the Zurich Center for Security Studies, says the increasing incidence of sabotage incidents underlines the risks in 2025's "interconnected" world.
"It doesn't take necessarily an outright missile," she says.
"We have come into a whole different sort of paradigm, at least when it comes to what state actors are willing to engage in."
As well as the risk to physical infrastructure, there are fears repeated acts of sabotage could undermine public confidence in governments to do their job in protecting the nation.
"This will inevitably raise the question from the society … about the competence of their government to do what governments are supposed to do," Ms Grgic says.
"That is to have the monopoly over the use of force."
Ms Grgic says the spate of attacks is leading to a rethink in the academic community about where the line between isolated incidents and a "full scale, above the threshold" war lies.
She warns these kinds of attacks could be viewed as a challenge to the limits of Article Five of the NATO agreement, a clause which stipulates that an attack on one is an attack on all members.
"We haven't come there yet," she added.
'Escalating' attacks a warning for Australia
Mr Landsbergis says Western leaders have so far been hesitant to respond to sabotage attacks, and "gave in" when red lines were crossed.
"Russia was escalating, we were de-escalating and it only increased their escalation.
"That is the main bad lesson we are learning."
Instead, he and other experts say NATO members needed to stop retreating and instead respond more forcefully to these acts.
Experts also caution that building resilience is key, both in terms of strengthening infrastructure, and in maintaining alliances like NATO.
They were also concerned that Russia's alleged actions could embolden others.
Already, similar undersea incidents have been reported in the South China Sea, where Beijing has long sought to stake a larger claim of the lucrative maritime shipping route.
Mr Landsbergis cautions that could expose Australia to saboteurs.
"If Russians consider themselves successful with their shadow activities in the Baltic Sea, it is safe to assume that others will learn from that," he says.
"Who knows — maybe we have to devise a strategy that would work not just in the Baltics, but in the region closer to you."
