Sonar image of object on floor of Baltic Sea. Credit: Peter Lindberg/OceanExplorer.se

Just what is that thing on the floor of the Baltic Sea? The treasure hunters who found it with side-scan sonar say all they know is that it's about 200 feet in diameter, and sits in about 275 feet of water between Sweden and Finland.

One reader pointed out that in the 19th century, a Russian naval architect named Andrey Popov experimented with disc-shaped ironclad warships. Two were built: the Novgorod and the Rear Admiral Popov. Popov apparently thought they would they would make stable gun platforms and operate well in shallow water. They were, instead, slow and hard to maneuver. They made their way into a 2007 book by Martin Dougherty, "The World's Worst Weapons."

The Novgorod was built in 1873 and scrapped in 1912. Take a look at a contemporary drawing:

A 19th-century artist's conception of the Russian ironclad Novgorod.

Peter Lindberg, one of the organizers of the effort in the Baltic, said they thought about Popov's ships and decided they were too small.

"The information we have got is that the largest of these odd round ships were approximately  35-37 meters [about 115-120 feet] across," he said in an email from Stockholm. "The circle is 55-60 meters across. Of course, there might be a possibility that the Russians constructed a larger round ship, which no one knows anything about today. If it is that, well then we have the explanation. I think, though, that it is a bigger chance that the circle being a natural phenomena rather than a Novgorod-ship, just my opinion."