The oldest wreck probably dates back to medieval times and could be up to 800 years old, while the others are likely from the 17th to 19th centuries, Peter Norman of Sweden's National Heritage Board said Tuesday.
"They could be interesting, but we have only seen pictures of their exterior. Many of them are considered to be fully intact. They look very well-preserved," Norman told The Associated Press.
Thousands of wrecks — from medieval ships to warships sunk during the world wars of the 20th century — have been found in the Baltic Sea, which doesn't have the ship worm that destroys wooden wrecks in saltier oceans.
The latest discovery was made during a search of the seabed east of the Swedish island of Gotland by the Nord Stream consortium, which is building a 750-mile pipeline in the Baltic Sea.
The 12 wrecks were found in a 30-mile-long and 1.2-mile-wide corridor, Nord Stream spokeswoman Tora Leifland Holmstrom said.
The heritage board said three of the wrecks have intact hulls and are lying upside-down at a depth of 430 feet.
Swedish marine archaeology experts analyzed pictures of the wrecks and determined that they could be of a high historic value.
"The content can tell us a lot about everyday life during that time," Norman said.
It's unclear whether any of them will be salvaged but the board said it hopes they will be explored by divers — though Norman added many of them are at a depth that would require very advanced and costly diving operations.
The Nord Stream consortium, which plans to start construction in April, has promised to make sure its activities don't damage the wrecks. The area where they were found is in Sweden's economic zone, but not in the planned route of the pipeline, Leifland Holmstrom said.
The Nord Stream project, in which Russia's OAO Gazprom holds a 51 percent stake, has uncovered scores of other objects during seabed searches of the route, including about 80 sea mines and a washing machine, she said.
Last year, parts of a 300-year-old ship were salvaged from Germany's Bay of Greifswald to clear a path for the pipeline, which expects to carry some 1.9 trillion cubic feet of natural gas a year.
Sweden's most famous maritime discovery, the royal warship Vasa, is housed in a popular museum in Stockholm where visitors can admire the ship's details, down to the flashing teeth of the carved lions that adorn its elaborate exterior. The Vasa was raised from the Stockholm harbor in 1961, 333 years after it sank on its maiden voyage.