An object, which some say resembles the Millennium Falcon, is raised about 10-23 feet above the seabed. The diving team found a 985-foot flattened 'runway' leading up to the object, suggesting that it skidded along the seabed before it stopped.
The Daily Mail reports that the object was first found a year ago, but divers were not able to revisit the site until recently due to a lack of funding.
However, the recent expedition has raised even more questions.
In a press release, diver Peter Lindberg said: "First we thought this was only stone, but this is something else."
He explains that the object has "rounded sides and rugged edges". It has a 60m diameter and, at the centre, has "an egg shaped hole leading into it from the top."
Surrounding the hole they found a strange, unexplained rock formation. The divers said that the rocks looked "like small fireplaces" and the "stones were covered in something resembling soot."
Mr Lindberg says: "Since no volcanic activity has ever been reported in the Baltic Sea the find becomes even stranger.
"As laymen we can only speculate how this is made by nature, but this is the strangest thing I have ever experienced as a professional diver."
The presence of soot was particularly confusing for Mr Lindberg's colleague, Stefan Hogeborn.
He says: "During my 20-year diving career, including 6,000 dives, I have never seen anything like this. Normally stones don't burn.
"I can't explain what we saw, and I went down there to answer some questions, but I came up with even more questions."
Mr Lindberg told Fox News: "We've heard lots of different kinds of explanations, from George Lucas's spaceship - the Millennium Falcon - to 'it's some kind of plug to the inner world', like it should be hell down there or something.
"We don't know whether it is a natural phenomenon, or an object. We saw it on sonar when we were searching for a wreck from World War I. This circular object just turned up on the monitor."
The diving company has now created a submarine, that they hope will appeal to tourists who would pay to take a trip 285 feet to the bottom of the Baltic Sea, to see the object for themselves.
Click on the image below to see video footage of the mystery object...