They know they're onto something big. If only they knew what that something was.

A group of treasure hunters based in Stockholm, using sonar, has found a strange disc-shaped object on the floor of the Baltic Sea between Sweden and Finland. From above, it looks a bit like the Millennium Falcon of "Star Wars" fame. It's large -- 197 feet in diameter -- and it's in about 275 feet of water. Leading to (or from) it is a churned-up track on the sea floor of about 1,600 feet.

What are the most realistic possibilities? "Let us put it like this: we have tried a lot of theories," Lindberg wrote in an email to ABC News. "The list is getting shorter and shorter with options, so for now we do not really know. We do not have anything that speaks more for one option or the other."

The treasure hunters would like to explore the wreck -- if it is a wreck -- with a small submarine, but they don't have the money to bring one in. If it were one of the conventional ships they've looked for in the past, they would hope to find gold or silver, but in this case they don't know what's actually there.

So they are talking to TV production companies, hoping one will fund them and make a documentary about their work. In the meantime, they are keeping the precise location of their find a secret, and waiting for spring and warmer weather.

"We are determined to successfully complete our mission of finding out what's at the bottom of the Baltic Sea," said Lindberg.