The group called Ocean Explorers and call themselves treasure hunters. Their mission is to comb the Baltic Sea wrecks containing alcohol which can then be sold at auction. 1997 found leader Peter Lindberg wreck Jonkoping in the Ã…land archipelago. On board were champagne that has subsequently been sold for millions of dollars.
But this morning they found something quite different.
"There's something solid down there"
At 87 meters, between Sweden and Finland, they saw a large circle, about 60 meters (197 ft) in diameter.
- You see a lot of weird stuff in this profession, but during my 17-18 years as a professional deep sea treasure hunter I have never seen anything like this. Shape, completely round the circle, makes this unique, said Peter Lindberg.
Next to the circle is a 300 meter long slide track - as if the circle has traveled across the seabed before it has settled.
- Germans have been there, the Russians have been there, dumped something huge, yes it is not inconceivable, said Dennis Ã…sberg involved in the expedition.
What could it have been?
- I have no idea. All I know is that I've seen something down there that is solid - stone, concrete or steel - and that is completely round.
They rule out theories that there is a depth bomb or mine from the First World War - or symmetrical algae bloom.
- I've eliminated those possibilities, this is not anything soft, says Peter Lindberg.
Andreas Olsson, marine archaeologist at the National Maritime Museum, looked at pictures of the circle and concluded it is too early to determine what it is:
- The most likely scenario is still that it is a natural ground formation.
it's a star gate