Determining the track of underground cables is generally difficult without special maps, which are barely available.

In case of the cable of HVDC SwePol ( http://en.wikipedia.org/wiki/SwePol ) near Ustka, Poland there are special markers marking its track ( see picture on http://www.panoramio.com/photo/24853707 )

Close to the track of the cable of HVDC Kontek ( http://en.wikipedia.org/wiki/Kontek ), which runs from Bentwisch in Germany to Bjaeverskov in Denmark, there are several cabinets with cable monitoring equipment. Please try to find out their locations.

Determing the track of an underground DC cable should be possible with a compass, when the cable is in operation.