Alarmed at the looting of historically valuable shipwrecks in the Baltic Sea, German archaeologists have started attaching underwater signs designating them as protected monuments.
Hobby divers and trophy hunters are damaging a precious maritime legacy stretching back thousands of years, they warn.
The two-man U-boat was discovered lying at a depth of 18 meters near Boltenhagen off Germany's Baltic Sea coast in 2000.
Its plexiglass turret hatch was intact and closed, which prompted authorities to designate it as a war grave because the crew of the vessel, of a type used by the German navy towards the end of World War II to evade Allied sonar detection and sink ships, was believed to still be inside.
Then someone dived down and removed the hatch in 2002. The local government responded by sealing the gap with a steel plate. But there have since been attempts to break it open.
"It's one of our big worries, over the years people keep trying to get into it and that is of course utterly disrespectful," says Detlef Jantzen, an archaeologist at the regional agency for monument protection in the northeastern German state of Mecklenburg-Western Pomerania.
The U-boat is one of some 1,500 marine monuments strewn across the seabed along the coast.
The area has a wealth of well-preserved shipwrecks, lost cargo, planes and even ancient settlements submerged through subsidence and rising water levels.
It amounts to a precious historical legacy and gives insight, for example, into boat-building techniques dating back to the Middle Ages and the events that led to the sinkings.