Why did Richard Burton and John Hanning Speke explore Lake Tanganyika?

Sir Richard Burton and John Hanning Speke explored Lake Tanganyika as part of their broader quest to discover the source of the Nile River.123

Following reports from missionaries and traders about a vast inland sea or "great lake" in the heart of Africa, the two explorers set out from Zanzibar in 1857 to investigate these claims. They reached the eastern shore of Lake Tanganyika in February 1858, becoming the first Europeans to do so.4512

While they initially sought the Nile's origin at the lake, their expedition was driven by the long-standing geographical mystery regarding where the river began. During their return journey, Speke traveled north independently and discovered Lake Victoria, which he subsequently identified as the true source of the Nile—a conclusion that led to significant professional disagreement and estrangement between him and Burton.413