The Baltic Sea is a graveyard of ships that have sunk over the past few centuries, in particular the two World Wars. German maritime expert Thomas Dehling has an eye on shipwrecks and sunken objects - and their exact positions.

DW: How many shipwrecked boats are there in the Baltic Sea?

Thomas Dehling: I don't know the figure for the entire Baltic Sea, but in German waters - about 16,000 square kilometers (6000 square miles), less than a tenth of the Baltic Sea - there are quite a lot, roughly 1,000 objects that we know about.

In the Baltic Sea, we have quite a lot of shipwrecks due to the two World Wars, more than in other seas. Most of the objects are from the early 20th century, but there is one shipwreck in Finnish waters that is more than 400 years old. In our part of the Baltic, there's an old wooden shipwreck that dates to the 15th or 16th century, the archeologists aren't sure. There's not much left but the cargo was still there: copper disks, a really unusual find so far north.

We regularly survey the objects, especially those that are potentially dangerous to shipping, depending on the importance of the object and the sea lane, maybe once a year or only once every 25 years.

How unusual would a Russian submarine shipwreck off the Swedish coast be?

There are several WWII Russian submarine shipwrecks in the Baltic Sea, that isn't unusual. In the case of the shipwreck off the Swedish coast, it depends on how old it is. Because of the wars, there are English submarines in German waters in the North Sea, a Polish shipwreck in the Baltic Sea, a German shipwreck in Polish waters. If the Russian submarine is not that old, it is obvious that a Russian submarine entered Swedish waters clandestinely.

Is the Baltic Sea popular with recreational divers?

Yes it is - and it's dangerous. Very often fishing nets get caught in the objects and you have to be very experienced to dive there. That's why we don't disseminate our information on the shipwrecks.

But there's a map on your website?

Yes, but you won't find an object when you use that map because it's not that precise. Of course we have to inform the maritime community about obstructions that might hinder their traffic, but we don't inform the public about objects that might be of historic interest - only people who can claim that they need to have the information.

Have you charted all of the objects in your sector by now, or do you still find new ones?

In our part of the Baltic Sea, we still find 10 to 15 objects every year that we didn't know about before, thanks to the modern technology on our ships that lets us survey the seafloor in much more detail. And many of the objects aren't large shipwrecks, but much smaller, or partially submerged. And they're not the ones that are potentially dangerous to shipping, we know about those. We don't remove anything, we inform the water and shipping administration, and it's up to them to decide whether to remove an object or set up a buoy.

Thomas Dehling heads the hydrographic surveying division at Germany's Federal Maritime and Hydrographic Agency (BSH), which provides maritime service for the shipping. The agency is based in Hamburg and Rostock.