aAt the end of an unmarked path on a small island on the edge of Stockholm's vast Baltic Sea archipelago, there is a small, unremarkable wooden hut painted dark red. The water gently hits the snow-capped rocks and the scent of pine fills the air.

The site offers few clues to the geopolitical drama that has gripped Scandinavia in recent months with accusations of infrastructure vandalism. But in reality, this cabin holds a vital cog of Europe's digital connectivity and a vulnerable point in a potential hybrid war. It's a data center that amplifies signals from a 1,615-mile fiber-optic cable that stretches from northern Sweden to Berlin.

Last month, two nearby fiber optic cables were severed, sparking an ongoing investigation by Swedish authorities. Western intelligence officials from several countries have said they believe a Chinese ship leaving the Russian port of Ustiluga caused the disconnection, but it was possible the disconnection was accidental or intentional. Opinions are divided as to whether this is the case.

Since Russia's invasion of Ukraine in 2022, Sweden has experienced an increase in hybrid warfare (attacks against adversaries using methods other than traditional military action) by pro-Russian factions. The Guardian was given exclusive access to the Stockholm data center site as Nordic governments are on high alert against Hybrid Russia's activities.

Daniel Aldstam, chief security officer at GlobalConnect, which transports 50% of Scandinavia's internet capacity and operates the center, described its approach to its location and normal appearance as “security through concealment.”

“There are basically two different approaches,” he said. “Either put a lot of fencing around it so it's obvious there's something important, or you can do what we've done here and make things a little more discreet. But , Of course, the usual facilities such as alarms, CCTV and access control are in place.'' Inside, cages full of equipment emit flashing lights and cables of different colors line the ceiling.

Following recent incidents of suspected sabotage, Polish Prime Minister Donald Tusk proposed a “naval security” initiative involving joint military patrols by countries around the Baltic Sea.

A helicopter ride from Stockholm over the archipelago of 30,000 islands, rocks and reefs highlights just how difficult it is to protect the coastline. But its vastness also suggests how the “security through concealment” approach can be effective, at least up to a point. A map has been published showing where all submarine cables are laid.

“We have hundreds of thousands of kilometers of fiber. How can I physically protect it? You can’t do that,” Aldstam said. “The key here is redundancy.” [using multiple cables offering alternative routes if one is cut off]. You need to eat more fiber. ”

Infrastructure is seen as particularly vulnerable to hybrid warfare, and there are signs of tweaks to the “covert” approach to reflect difficult times.

GlobalConnect is building a larger, more modern-looking data center nearby. This data center is still unmarked and painted in a similar shade of red, but it is a building that clearly serves a more important function. It has an internal diesel-powered backup generator that allows it to continue operating even if the electricity is cut off.

Since Russia's full-scale invasion of Ukraine, the vulnerability of submarine cables and other critical infrastructure to sabotage has been highlighted, particularly in the relatively shallow and busy Baltic Sea region.

In September 2022, the Nord Stream pipeline carrying natural gas from Russia to Germany exploded. Initially, many thought Russia was responsible. But in August, German media reported that German authorities had issued an arrest warrant for a Ukrainian man suspected of being part of the team that planted explosives on the pipeline. Both sides in the Ukraine war deny responsibility and blame each other for the attack.

NATO, which has established a dedicated center for undersea security, says vulnerabilities in the infrastructure of wind farms, pipelines and power cables put the security of around 1 billion people in Europe and North America at risk of hybrid warfare by the alliance's adversaries. He warned that he was being exposed. Earlier this month, NATO Secretary-General Mark Rutte urged Europeans to “shift to a wartime mindset.”

Despite all the warnings, the number of undersea cables lying or buried on the ocean floor appears to be surprisingly small.

“We call it a super-mega cable, but it doesn't sound super-mega, and it doesn't look super-mega,” said the director of Global Connect's entire Sweden-to-Berlin cable project, which was completed earlier this year. Director Patrick Gailshaw said. “The name refers to capacity, not size.”

Inside the cable, which is just over 2cm in diameter, is a small section made up of 96 hair-thin fiber pairs, enough to support 1 billion simultaneous Netflix streams. The rest consists of a steel cladding and waterproof material.

Gailshaw said a relatively small ship's anchor would be enough to destroy the cable. “You don't need a super big tool to break or cut this cable. It's pretty fragile.”

Making it more powerful means it's heavier, more expensive and “more complex to deploy,” he added.

Accidental breaks in submarine cables are extremely rare. “It is very rare for damage to occur in general,” says Gylesjö. “It's very unusual. I think something like this has happened up to two or three times during our time as submarine cable operators.”