In a time without GPS and echosounder, European engineers and black labourers worked for decades to get a better understanding of the nautical intricacies of the Congo River. This is the first comprehensive story, in text and custom-made maps, of the, in flow, second largest river in the world. We follow the earliest explorers mapping the river, the expeditions to find an alternative access to the ocean and the first land and hydraulic surveys to improve navigation. The constant movement of shallows and islands keeps the guardians of the river constantly on their toes. Over the years, better technologies on all fronts improved safety, data collection and fairway maintenance. In conclusion, the author describes a proposal to develop a 21st container port that would rival any port facilities on the African west coast.