Lindberg discusses various possibilities for what the object might be: "It has these very strange stair formations, and if it is constructed, it must be constructed tens of thousands of years ago before the Ice Age," he said in the radio interview. (The peak of the most recent Ice Age occurred some 20,000 years ago.)

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Lindberg acknowledges that the object could instead be a natural formation, such as a meteorite that penetrated the ice during the Ice Age, or an underwater volcano; however, he gives the impression that scientists are baffled by it. Geologists, for example, have supposedly told him the object "cannot be a volcano."

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The divers recently gave samples of stone from the object to Volker Brüchert, an associate professor of geology at Stockholm University. Swedish tabloids quote Brüchert as saying: "I was surprised when I researched the material I found a great black stone that could be a volcanic rock. My hypothesis is that this object, this structure was formed during the Ice Age many thousands of years ago."

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 "It's good to hear critical voices about this 'Baltic Sea mystery,'" Brüchert wrote in an email. "What has been generously ignored by the Ocean-X team is that most of the samples they have brought up from the sea bottom are granites and gneisses and sandstones."

These, he explains, are exactly what one would expect to see in a glacial basin, which is what the Baltic Sea is — a region carved out by glacial ice long ago.

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Aside from a widely-reproduced illustration recently created by a graphics artist in which the Baltic seafloor object is rendered as a beautiful, Atlantis-like archaeological site, there has only ever been one actual image of the Baltic Sea object: the original sonar scan image captured by the divers last summer, in which the object resembles a crashed UFO spaceship. But experts told us that sonar image should be disregarded.

'Mysterious' Baltic Sea Object Is a Glacial Deposit