The unidentified circular object that has held the world in a viral grip for a year probably isn't an alien spacecraft after all.

Over the past few days, divers have photographed the unusual anomaly on the seabed nearly 300 feet below the surface.

According to news released Friday by the OceanX treasure hunters who discovered the anomaly on June 19, 2011, "first they thought it was just stone or a rock cliff, but after further observations, the object appeared more as a huge mushroom, rising 10-13 feet from the seabed, with rounded sides and edges.

"The object had an egg-shaped hole leading into it from the top, as an opening. On top of the object they also found strange stone circle formations, almost looking like small fireplaces. The stones were covered in something resembling soot," reports the OceanX website.

Baltic Sea UFO Turns Out Not To Be A UFO.

One of the OceanX divers, Stefan Hogeborn, a veteran of more than 6,000 dives in his 20-year career, is completely puzzled by what he saw.

"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 questions, but I came up with even more questions," Hogeborn said.

The first pictures of the Baltic Sea object have been published by the Swedish newspaper Expressen, which reported the "images show that the circle consists of several blocks formed by 'rolls' or 'mushrooms' that are attached to each other, forming the circle."

On another dive, a sledgehammer was used to obtain samples of the material. "Since we did not get any answers to the questions we asked ourselves, we have brought this to the experts who may be looking at the pieces we brought up," said Hogeborn.

What does the Baltic Sea UFO look like close-up?

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  • Melbourne, Australia, Feb. 2013

    This is a composite image of how three alleged UFOs maneuvered about in the sky over Melbourne, Australia, in early February, 2013. The final verdict isn't in yet on whether they're birds, aircraft, balloons, bugs or something truly unidentified.

  • Lanterns

    These candle-lit Chinese lanterns can rise high into the sky and are often mistaken for UFOs.

  • Exploding Weather Balloons, Not UFOs

    On Dec. 20, 2012, a bright, circular object (pictured at the top of this composite image) was videotaped exploding in the skies above Sacramento, Calif. It wasn't immediately identified, resulting in speculation that it was either an alien spacecraft, military top secret weapon, runaway planet, North Korean satellite, among others. Within a short period of time, it became apparent that this was a weather balloon. The bottom part of this image shows such a balloon as it ascended over Tampa Bay, Fla., on July 2, 2012, and exploded in an identical manner as the Sacramento object, probably much to the dismay of all true ET believers out there.

  • Boomerang UFO composite images -- 10-5-12

    This is a composite of images shot by two eyewitnesses of a boomerang-shaped UFO they reported seeing over their Burbank, Calif., home on Oct. 5, 2012. Mutual UFO Network photo/video analyst Marc Dantonio concluded the object was likely "a balloon, floating on the wind that has collapsed in half."

  • Changing UFO Pattern -- Warren, Mich. 1-10-13

    This four-image series of lights in the sky was recorded over Warren, Mich., on Jan. 10, 2013. The lights were seen changing into several patterns. The most logical explanation for these types of UFOs is a series of balloons or lanterns.

  • UFOs Over Earth

    This composite image shows four different times that alleged UFO were photographed above Earth by either space shuttles or the International Space Station. The big question is whether or not they are truly unidentified objects or if they are more likely reflections from spacecraft windows, meteors or fast-moving spacecraft-generated debris.

  • Pink UFOs Or Lens Flares?

    What appear to be pink-red UFOs are actually lens flares from the Google Earth street view camera as it snapped images in Texas (left) and New Mexico (right).

  • Lens flares Arizona

    These two flying saucer-shaped, pink-colored lens flares were created by the Google Maps camera as it drove through locations in Sedona, Ariz. (left) and Flagstaff, Ariz. (right). The images were snapped in April 2009. Submitted to HuffPost by trenna.

  • Lens flare Whiteriver, Ariz.

    This skybound lens flare was created by a Google Maps camera in June 2008 over Whiteriver, Ariz. Submitted to Huffington Post by Cheryl Weeks.

  • Lens flare Gulfport, Miss.

    This very Earthbound lens flare was created by a Google Maps camera in November 2007 at Gulfport, Miss. Submitted to Huffington Post by Jenni Parker.

  • Lens flare Eureka Springs, Ark.

    This seemingly grounded lens flare was created by a Google Maps camera in January 2008 at Eureka Springs, Ark. Submitted to Huffington Post by SE.

  • Lens flare Escanaba, Mich.

    This lens flare appears to be following a car. The Google Maps image was created in October 2008 at Escanaba, Mich. Submitted to Huffington Post by Mary Robinson.

  • Cincinnati Skydivers NOT UFOs Sept. 28, 2012

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  • Clouds

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  • Sunken Ship in the Baltic Sea

    On June 19th the Swedish-based diving company Ocean Explorer discovered something they've never quite seen before. They were exploring in the Baltic Sea between Sweden and Finland looking for sunken treasures when a very unusual image suddenly appeared on the sonar. A 197 feet diameter cylinder shaped object was discovered at the depth of approximately 275 feet which resembles the Millennium Falcon from the movie Star Wars.

  • Baltic Sea UFO 1

    An image released on June 15, 2012, shows a close-up view of the unidentified object sitting on the bottom of the Baltic Sea.

  • Baltic Sea UFO 2

    Close-up of rock bed that forms the Baltic Sea UFO, which still mystifies researchers.

  • Baltic Sea UFO 3

    One of several odd stone circle formations, sitting on top of the unidentified object at the bottom of the Baltic Sea.

  • Antarctic UFO -- Aug. 10, 2012

    A circular UFO hovers above the Neumayer-Station III research facility in Antarctica on Aug. 10, 2012. Theories ranging from a simple weather balloon to a more elaborate ship from another planet have run the Internet gamut. The next slide shows a closeup of the object.

  • Antarctic UFO Closeup -- Aug. 10, 2012

    This is a closeup of the UFO from the previous slide. No official explanation has been offered about the object.

  • Manufactured UFO -- 2011

    Pictured is a quad copter -- a deliberately manufactured UFO created by special effects wizard Marc Dantonio for a National Geographic special, "The Truth Behind: UFOs," which aired in December 2011. On the left is what the small device looks like resting on the ground, measuring 4 feet in circumference. At right, is how it appeared behind a tree in the night sky.

  • Police Dashboard Camera In Texas

    In February of 2012, this fireball was captured by a Texas police chief's A dashboard camera. F.A.A. say this was probably a meteor, falling to Earth. .

  • Meteors

    Meteors: Space debris can create a spectacular light show when it burns through the Earth's atmosphere, and sometimes reported as UFOs.

  • Civilian or Military Aircraft

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On-site researchers are still scratching their heads at the photographic evidence.

Treasure hunters Peter Lindberg and Dennis Asberg first discovered the jumbo jet-sized object -- variously described as disc-shaped, circular and cylindrical -- using side-scan sonar. When images and videos were released on the Internet, the story quickly went viral and speculation took off as to the identity of the object, which resembles the fictitious Millennium Falcon spacecraft from the "Star Wars" movies.

Everything from sonar glitches, a sunken Russian ship, naturally occurring stone outcroppings and, of course, alien spaceship have been debated over the past year.

On June 1, Lindberg and Asberg headed back to the location with deep-sea divers and remote-controlled cameras.

"We need to explore what we have found," Lindberg, captain of the Ocean Explorer, said on the treasure hunters' website. "Media has been speculating about everything from UFOs to Russian space ships, and the time has come to find out what the sonar image actually is."

Isn't it also about time that we all agree to refer to the anomaly as a USO, instead of UFO? USO, as in Unidentified Submerged Object. Because, in reality, nobody knows if this thing was ever in the air, flying, right?

"It was originally claimed to be a perfectly round disc, and of course, that's one of the things that made it a mystery," said Benjamin Radford, deputy editor of Skeptical Inquirer magazine. "If it's a spaceship, as some have suggested, it must be almost entirely buried in the ocean floor," Radford told The Huffington Post.

On June 11, posted this message from expedition co-leader Asberg: "Everything is top secret now ... because of the risks ... hope you all understand this is no game. But the truth will be reported shortly."

Now that the team has almost certainly revealed that the mushroom-shaped anomaly is not from another planet, they're still not exactly sure what it is.

"First we thought this was only stone, but this is something else," said Lindberg on the OceanX website. "And 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."

Clarification: The headline of the initial version of this story gave the impression that the Baltic Sea anomaly is an actual mushroom. The story correctly quoted the on-site investigators who described the submerged object appearing like a mushroom. As this story progresses and analysis is conducted on samples of the undersea object, updates will be forthcoming.

Sonar image and artist rendering of the Baltic Sea USO.