Scientists have found themselves stumped by what appears to be an "alien spaceship" or "Nazi bunker" deep under the waves of the Baltic Sea.

Marine explorers identified a structure in the sea between Sweden and Finland but over a decade on from the discovery boffins still find themselves mystified at what it could be.

Sonar images found the underwater anomaly that appears to show an underwater structure, which has confused experts who are not quite sure what the Baltic Sea Anomaly is, The Mirror reported.

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Featuring a strange "1,000-foot long runway" and a strange object that appears to have slid across it, the Baltic Sea Anomaly has seen divers take closer looks.

Scientists are stumped over the origins of the mysterious Baltic Sea Anomaly (stock)
Scientists are stumped over the origins of the mysterious Baltic Sea Anomaly (stock)

What was found below by the divers were steps and compartments, the potential for tunnels and passageways beneath the surface of the unknown object and an ability for the object to block sonar.

Those heading below the surface to snap some photographs of the interesting item found their equipment was not in working order, with sensor and photographic equipment both failing.

Mysterious electrical interferences were cited as causing trouble, draining the recently-charged batteries and making it impossible to get a closer look.

Outlandish theories live on, not just that it may be an extra-terrestrial spacecraft which landed in the depths of the ocean, but that it may be a secret Nazi bunker.

Theories over the presence of the anomaly have seen claims of Atlantis, aliens and Nazis (stock)
Theories over the presence of the anomaly have seen claims of Atlantis, aliens and Nazis (stock)

Simon Andrews, who first wrote the suggestion the bunker could be an alien device, has noted it could be a secret military project pushed by Adolf Hitler during World War Two.

Claims a bunker was lowered into the sea were aired, but other claims over the presence of the bunker, including that of an ancient temple or Atlantis, have also been noted.

There is also the chance the Baltic Sea Anomaly, which has stumped experts around the globe, is nothing more than a glacial deposit.

Volker Brüchert, an associate professor of geology at Stockholm University, said: "My hypothesis is that this object, this structure was formed during the Ice Age many thousands of years ago."

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