Sub-bottom systems able to distinguish poles less than 10 cm in diameter embedded in the sea-floor sediments have been used since 1994 to map submerged archaeological pole structures, such as harbours and defensive barrages intended to protect against naval attacks, located in relatively shallow water. This approach has proved its worth in providing fast and cheap large-scale information about the horizontal configurations of such structures, making it possible to distinguish and target zones that, with excavation, can elucidate central archaeological problems. For instance, this method has permitted the identification of repeated repair phases in large-scale constructions, which would have been extremely time-consuming and much more expensive to distinguish and map in the conventional way, with an excavation carried out by divers. A precondition for success is precise positioning of the recorded features, allowing a diver subsequently to be directed to them with a real precision (not a statistical one) of a few decimetres. This paper presents some examples of the application of this technique from several central archaeological sites dating from the Iron Age, Viking Age and the Medieval period, such as the harbours at Haithabu, Germany, and Vordingborg and Jungshoved, Denmark, as well as barrages against naval attack located in Haderslev Fjord, Kerteminde Fjord and Jungshoved Vig, Denmark. It discusses cost-effective verification strategies, including collection of samples for radiocarbon dating, dendrochronological dating etc. The science explaining how the poles can be distinguished acoustically is also discussed.