In June 2011, marine surveyors Peter Lindberg and Dennis Asberg were mapping the northern Baltic Sea floor when sonar detected a large, circular object roughly 60 meters across. The shape had unusually sharp edges, and a straight, 300-meter trench extended from one side of it across the seabed. The discovery puzzled the team, which had years of experience locating shipwrecks and recovering cargo from the region. They had found nothing comparable in their previous work.

The object, which became known as the Baltic Sea anomaly, sits at a depth of about 85 to 90 meters. It appears to rise several meters above the surrounding seafloor. A year after the initial scan, the team returned with better equipment. When their vessel approached within about 200 meters of the object, all electronic systems failed. Satellite phones, GPS, sonar, and cameras stopped working simultaneously. When the vessel moved away, the systems restored themselves. The crew repeated the approach several times and observed the same pattern each time.

That electronic shutdown was not an isolated incident. Reports from later expeditions, including a university mission in 2025, described the same failure of navigation equipment near the site. The cause of this interference has not been identified.

During a 2012 expedition, divers descended to examine the object directly. Visibility at that depth was limited to roughly one square meter due to suspended sediment. Lead diver Stefan Hogeborn reported seeing vertical walls that met at right angles, a feature not typical of naturally eroded rock. He also described a circular hole about two meters in diameter, surrounded by a raised square rim, on the object’s upper surface. Smaller holes were present in some sections of the material.

Divers described the surface as rough, pitted, and blackened in places, with some areas resembling cooled molten material. Hogeborn noted that the water temperature directly at the object appeared colder than the surrounding water, although this measurement has been difficult to verify consistently.

Rock samples collected from loose debris around the site were sent to Stockholm University, where geologist Volker Brüchert identified them as basalt. Basalt forms from cooled lava. The Baltic region sits on the Fennoscandian Shield, a geological formation that has not experienced volcanic activity for hundreds of millions of years. Brüchert concluded that glaciers likely transported the material from a distant volcanic source during the last ice age.

Other geologists examined the samples and reached differing conclusions. Some questioned whether the material could have formed entirely through natural seabed processes. No core sample has ever been taken from the main body of the structure itself, so its internal composition remains unknown.

Several scientists have proposed that the anomaly is a glacial deposit, possibly a drumlin, formed by the movement of ice sheets. That explanation assumes the object is a natural outgrowth of the seafloor geology. In June 2025, a Stockholm University research vessel reportedly used sub-bottom profiling technology, which can map geological layers beneath the seafloor, to examine the site. According to Asberg, the scan showed the anomaly sitting on top of the seabed, separated from the geological layers below it. If confirmed through peer review, that finding would be difficult to reconcile with the glacial deposit hypothesis.

The location of the anomaly is also relevant to what is known about the region’s human history. The Baltic Sea formed roughly 8,500 years ago, when melting glaciers flooded land that had previously been dry. For thousands of years before that, the area was inhabited by hunter-gatherers.

In 2024, archaeologists published evidence of a human-built stone structure in the Bay of Mecklenburg, in the southwestern Baltic. The structure, called the Blinkerwall, consists of nearly 1,500 stones arranged in a line roughly one kilometer long. Researchers believe it was built more than 10,000 years ago to guide reindeer herds toward a hunting area. It was later submerged when sea levels rose. The discovery confirmed that large artificial stone structures can exist on the Baltic seabed and remain hidden for millennia.

The Baltic Sea anomaly continues to attract attention, but no consensus exists about what it is. The available evidence includes unusual sonar imagery, reports of electronic interference, and rock samples with an unexpected geological profile. The 2025 scan data has not yet been published in a peer-reviewed journal. The scientific mainstream continues to treat the anomaly as a probable natural formation. Without a core sample from the structure itself, its origins remain uncertain.