Why was the European discovery of Lake Tanganyika significant for the search for the Nile’s source?

The European discovery of Lake Tanganyika by Sir Richard Burton and John Hanning Speke in 1858 was a pivotal moment in the search for the Nile River’s source because it served as the catalyst for further exploration of the African interior.12

Following reports from missionaries and traders about a "great inland sea," the Royal Geographical Society sponsored Burton and Speke to locate this body of water, which was hypothesized to be the origin of the Nile. While the expedition successfully reached Lake Tanganyika, the discovery shifted the focus of the search when Speke, on the return journey, traveled north and identified Lake Victoria.123

The disagreement between Burton and Speke regarding whether Lake Tanganyika or Lake Victoria was the true source fueled intense scientific debate and prompted subsequent expeditions, such as those by Henry Morton Stanley, which eventually mapped the complex drainage systems of the region and definitively resolved the question of the Nile's origins.123