The Iatmul believe that they all originated from a hole in the ground in Sawos (Gaikundi) territory. Other oral traditions tell of drifting down the river on rafts, having started somewhere in the west. The Sepik Basin is, from the point of view of geology, relatively young, having achieved its present character around 1,000 years ago. The whole area was flooded by the sea until about 5,000 years ago; only gradually, when the coastline withdrew until it reached its present location, did the alluvial plains form and marine conditions change to those of fresh water. Linguistic and archaeological evidence suggests that the Ndu speakers came down into the Sepik Basin from a southern tributary. The Sepik River (called the Kaiserin-Augusta-Fluss during German colonial times) was a main passageway for colonial administrators traveling upriver by ship. During German rule the first official Sepik exploratory expedition took place in 1886, and it was followed by Several others. After World War I, when Ambunti Patrol Post was established, the new Australian administration tried to suppress head-hunting. They finally succeeded in the mid-1930s by publicly executing convicted Iatmul warriors. The pacification of the Iatmul—a culture in which much emphasis was placed on male aggression and head-hunting raids—brought far-reaching cultural change from the outside world. Iatmul villages were in continuous contact with neighboring groups to the north and south, often in a symbiotic Subsistence relationship with the Iatmul trading turtles and fish in exchange for sago. The Sawos were regarded as nurturing mothers in this regard. Women conducted the trade while men were involved in joint rituals with neighboring groups.