STOCKHOLM A dozen centuries—old shipwrecks - some of them unusually well—preserved - have been found in the Baltic sea by a gas company building an underwater pipeline between Russia and Germany, Swedish experts said Tuesday.

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, said Peter Norman, of Sweden’s National Heritage Board.

“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,” Mr. 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.

Sweden’s most famous 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 harbour in 1961, 333 years after it sank on its maiden voyage.

The latest discovery was made during an analysis of the seabed east of the Swedish island of Gotland by the Nord Stream consortium, which is building a 750—mile (1,200—kilometer) pipeline in the Baltic Sea.

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,” Mr. Norman said.

The 12 wrecks are in Sweden’s economic zone, but not in the planned route of the pipeline, the Swedish heritage board said. Nord Stream, which plans to start construction in April, has promised to make sure its activities don’t damage the wrecks, it said.

The heritage board said three of the wrecks have intact hulls and are lying upside—down at a depth of 430 feet (130 meters).

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.

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