Baltic Sea thing

Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn! The Baltic Sea thing is an unusual structure, most probably natural, on the bottom of the Baltic Sea. It still has no "official" name. It has been called the Baltic (Sea) Anomaly by people who like the word "anomaly", and the Baltic Sea UFO by UFO nuts and tabloid journalists.

It was discovered in the summer of 2011 during a sonar survey of the sea floor by a team of marine treasure hunters looking for a wreck to pillage salvage. The sonar showed an unusually regular circular formation with a diameter of about 60 meters (200 feet).

Dives in 2012 found out that it was a stone dome resembling a mushroom.

The most likely explanation is some kind of rare natural structure, such as:
 * Something analogous to a
 * An inactive
 * Glacial detritus from the last ice age, a

Various unlikely explanations have been also proposed:
 * A (a "popovka"). This is right out, as only two were ever built and both ended up scrapped
 * An extraterrestrial spaceship, though this was pretty much debunked by the 2012 dives which established that it was made of stone
 * An extraterrestrial spaceship made of stone (Take that, skeptics!)
 * A prehistoric archaeological site submerged due to rising sea levels (Unlikely, as the Baltic basin was heavily glaciated during the last ice age, thereby effacing any evidence of human occupation. The great weight of ice depressed the land mass so that it was replaced with water when the ice melted. Currently the Baltic basin is actually and sea level is therefore effectively falling. )
 * A remnant of a prehistoric super-civilization (really unlikely)
 * Cthulhu's resting place (really, really unlikely )