Professional diver Peter Lindberg has pulled up a lot of things off the sea floor in his career. For example, the Swede once recovered a 100-year-old champagne collection from a shipwreck. But even with all his experience, he's never seen anything like this. He's baffled. What is "this?" Think of it as a U.F.O -- an unidentified floating object of sorts (it's really a sunken one).
"I have been doing this for nearly 20 years so I have a seen a few objects on the bottom, but nothing like this," Lindberg told CNN.
While doing a routine dive at a "secret location" (that's what CNN calls it) between Sweden and Finland, the deep-sea salvage company Ocean Explorer noticed something incredible located more than 80 meters down.
"We had been out for nine days and we were quite tired and we were on our way home, but we made a final run with a sonar fish and suddenly this thing turned up," he added. He even used the term "U.F.O" to help describe it.
Here's how CNN describes it:
Using side-scan sonar, the team found a 60-meter diameter cylinder-shaped object, with a rigid tail 400 meters long.
The imaging technique involves pulling a sonar "towfish" -- that essentially looks sideways underwater - behind a boat, where it creates sound echoes to map the sea floor below.
On another pass over the object, the sonar showed a second disc-like shape 200 meters away.
Lindberg's team believe they are too big to have fallen off a ship or be part of a wreck, but it's anyone's guess what could be down there.
The Head of Archaeology at Sweden's Maritime Museums, Andreas Olsson, admits he's intrigued by the picture, but remains sceptical about what it could be.
The reliability of one-side scan sonar images is one of his main concerns, making it difficult to determine if the object is a natural geological formation or something different altogether.
So what theories are circulating?
"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," Lindberg said. Others have speculated it's some sort of early Russian explorer vessel.
"But we won't know until we have been down there."
The discovery first made news this past summer but is resurfacing now because the team still hasn't figured out what it is.
For now, Lindberg and his crew are leaving the site alone until they can return in spring when the waters are calmer. They'll try to figure out what it is then. But even if they don't, the site could still prove valuable.
"The object itself is maybe not valuable in the sense of money it can be very interesting whatever it is, historical or a natural anomaly," said Lindberg.
And you can be sure people will flock to see it.
Do you have any theories?
This story has been updated for clarity.