The underwater treasure trove of nautical antiquities was discovered during a probe of the sea bed to prepare for the installation of a large gas pipeline.

“We have managed to identify 12 shipwrecks, and nine of them are considered to be fairly old,” Peter Norman, a senior advisor with the heritage board, told AFP.

“We think many of the ships are from the 17th and 18th centuries and we think some could even be from the Middle Ages,” he said, stressing that “this discovery offers enormous culture-historical value.”

The shipwrecks were discovered during a probe by the Russian-led Nord Stream consortium of the sea bed route its planned gas pipeline from Russia to the European Union will take through the Baltic.

“They used sonar equipment first and discovered some unevenness along the sea bottom … so they filmed some of the uneven areas, and we could see the wrecks,” Norman explained.

The discovery was made outside Sweden’s territorial waters, but within its economic zone, he said.

None of the wrecks were in the actual path the Nord Stream pipeline is set to take, but they were in its so-called anchor corridor, meaning they are in the area where ships laying the pipeline might anchor, Norman said.

“That’s one of the reasons this probe was done: to avoid damaging wrecks on the sea bed,” he said, adding that the Swedish National Heritage Board had received assurances from Nord Stream that “the positioning of the wrecks will be taken into account when they lay the pipeline”.

Due to its low temperatures and oxygen levels, the Baltic Sea is known as an ideal environment for conserving shipwrecks, which can remain virtually unblemished for hundreds and even thousands of year.

According to Norman, some 3,000 shipwrecks have been discovered and mapped in the Baltic, but experts believe more than 100,000 whole and partial wrecks litter the sea bottom.

“What makes this discovery so unique is that these wrecks have their hulls fully intact,” Norman said, adding however that there were no plans to raise the wrecks, which lie at a depth of more than 100 metres.