The object, located on the sea floor between Sweden and Finland, is some sort of "natural, geological formation," Peter Lindberg, the leader of the Ocean Explorer team, told FoxNews.com.
Lindberg and other scientists and divers spent 12 day exploring the 200-foot-wide roughly circular object under the Baltic they had first seen in sonar scans a year ago.
"It's not obviously an alien spacecraft. It's not made of metal," he said.
Lindberg concedes that it could be an alien space ship but only if aliens chose to construct their spacecraft out of meteor-like rocks.
"Who says they had to use metal?" Lindberg joked. "This trip has raised a lot of questions."
The central object appears to be a giant stone, "the kind divers see in keys and harbors," Lindberg said.
"There are other, loose stones lying around as well," he added. "The formation of rocks is 60 meters (200 feet) in diameter."
Strangely, there appears to be a pillar holding up the 200-foot-wide object, Lindberg said.
"We're going through the footage right now," Lindberg said.