During excavation works for the construction of an underground network below the Waterlooplein, a square in the centre of Amsterdam, some rounded unweathered boulders of Scandinavian origin were found. These boulders have been deposited here by human activity. The material originates from rocks in southeast and middle Sweden. It is believed that the boulders are transported by glaciers during one of the later pleistocene glacials to the coastal area of Blekinge in southeast Sweden where they became rounded at a later stage. In the Middle Ages and also afterwards ships sailing from Karlskrona to Amsterdam presumably carried these boulders as ballast.