When Sweden joins NATO, it will help the alliance repair its vulnerability in northwestern Europe: the Baltic Sea, a waterway shared with Russia that constitutes a bottleneck for access to the ports of eight states, including Germany.

Sweden’s key to maintaining navigable waters in the event of conflict is its world-leading submarine fleet, which analysts say boasts some of the most advanced conventional submarines ever built.

“The Swedish submarine fleet is well prepared for this environment and will greatly contribute to NATO’s overall submarine capabilities in the Baltic,” a NATO official told Reuters.

The Baltic, called a “flooded meadow” by some in marine circles, has an average depth of about 60 meters, making it too shallow for the nuclear-powered submarines that make up most of the Russian submarine fleet and the entire United States.

Sweden has three advanced Gotland-class submarines and an older model that will be retired when two newly designed A26 vessels are delivered in 2027 and 2028, bringing it to five by the end of the decade.

What Sweden's submarines bring to NATO.

Q: Why are the Swedes so strong?

The experience. Sweden has been operating submarines in the Baltic since 1904. None of the neighboring countries have been as active underwater as the Swedes.

“We have regional experience, which fills a gap, experience that NATO does not have,” Submarine Flotilla Commander Fredrik Linden said.

The Baltic is also a complex waterway: With many rivers feeding it, its salinity levels vary greatly. This changes both the buoyancy of a submarine and the way sound travels underwater, and local knowledge is needed to navigate successfully.

Swedish submarines can stay submerged for weeks. While submerged, conventional submarines run on batteries. Most need to return to the surface after a couple of days so that their diesel engines can work and recharge their batteries.

But Swedish subs have liquid oxygen stored in tanks on board to run diesel engines underwater and recharge batteries, so they can stay submerged longer and reduce the risk of detection.

What Sweden's submarines bring to NATO.

Q: How will it help in the future?

In 30 or 40 years, or perhaps sooner, submarine warfare will probably be autonomous, says Sebastian Bruns, a senior fellow at the Institute for Security Policy at the University of Kiel. In anticipation of this, Sweden has ordered two new submarines, with delivery scheduled for 2027 and 2028.

Known as the A26, the ships built by SAAB Kockums will be larger and more versatile than the Gotlands and will have a unique feature: a 1.5-meter-diameter plunge lock called a multi-mission portal at the bow.

This will allow remotely operated vehicles (ROVs), autonomous vehicles or groups of divers to enter and exit easily. According to Bruns, this makes the submarine ideal for seabed warfare, for example, protecting or destroying oil pipelines or other critical infrastructure on the seabed.

“Seabed warfare is the hottest topic in naval circles right now,” Bruns says, referring to the 2022 explosions that ripped apart the Nord Stream gas pipelines that run under the Baltic.

ROVs can perform tasks such as retrieving or placing objects on the seabed, scanning large areas, or laying or destroying mines. They can also dive deeper than normal submarines.

Johan Ahlander