The team of four divers was led on July 6 by Christian Ekström, who had a tip from a fisherman that there was a wreck at the site.
The wreck, which is at a depth of 55 metres, is in good condition and contained a number of intact bottles.
Ekström guessed they might contain champagne. He took one with him to help identify the age of the wreck. The shape of the bottle indicates that it is from the 1780s.
Opening the bottle, Ekström found that it tasted it like champagne. He offered tastes to several wine experts, who were highly impressed.
Sommelier Ella Grüssner Cromwell-Morgan told the newspaper Ålandstidningen that the wine is amber-coloured and full of rich, mingled aromas. "There was a lot of ripe fruit, overtones of yellow raisins and plenty of tobacco aroma. This is something one dreams of. I'm very grateful that Christian came to ask my opinion. I still have a little left in my refrigerator which I keep going back to sniff!"
Swedish champagne expert Richard Juhlin says that the bubbly is 98 percent certain to be from the French house of Veuve Cliquot, which was established in 1772.
Juhlin says that if so, it could be a real sunken treasure, perhaps worth more than 50,000 euros a bottle. So far the world's oldest known drinkable champagne dates back to 1825. It was unveiled with great fanfare in France in March 2009.
However under Åland's laws, the contents of shipwrecks from more than a century ago are the property of the province. So it is unlikely that the divers will be able to keep it or sell it to wine collectors.
"We can look into all the formalities later," says Viveka Löndahl, head of the province's museum board. ""Now we need to study and document the wreck further." She stressed that divers must have a permit to explore the site.
It remains unclear how many bottles -- or other possible treasures -- lie in the wreck.