A dozen centuries-old shipwrecks -- some unusually well-preserved -- have been found in the Baltic Sea by a gas company building an underwater pipeline between Russia and Germany.

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 the National Heritage Board yesterday.

"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."

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 1,200- kilometre pipeline in the Baltic Sea.

The 12 wrecks were found in a 48-kilometre-long by 1.2- kilometre-wide corridor, said Nord Stream spokeswoman Tora Leifland Holmstrom.

The heritage board said three of the wrecks are lying upside-down at a depth of 130 metres.

Swedish marine archeology experts analyzed pictures of the wrecks and determined they could be of a high historic value.

It's unclear whether any of them will be salvaged, but the board said it hopes they will be explored by divers.