By this time, the reports of early explorers like Rebmann and Krapf encouraged the British Royal Geographical Society to send an expedition to east Africa to search for the source of the White Nile. The leaders were Lieutenant (later Sir) Richard Francis Burton and Lieutenant John Hanning Speke.

In December 1856, on the last day of mourning for Sultan Said, Burton and Speke arrived in Zanzibar. They sailed for Bagamoyo and followed the slave route towards Lake Tanganyika, which they hoped was the source of the Nile. When they arrived, in January 1858, local Arab traders told them that a river at the northern end of the lake flowed into the lake (not out of it). Burton and Speke were unable to reach the point where the river met the lake.

Bitterly disappointed, they began to return eastwards to Zanzibar. Burton became ill, and was forced to stop, so Speke struck out northwards on his own, and became the first European to see the great nyanza (meaning 'lake') which he named Lake Victoria, certain it was the source of the White Nile – although he was unable to prove it at the time. Speke and Burton returned to Zanzibar in March 1859, and then separately to London.

To verify his theory, Speke returned to Zanzibar in 1860 with the Scottish explorer James Grant. Together they travelled inland to Lake Victoria, and this time found a great river emptying Lake Victoria at a waterfall, which they named the Ripon Falls after the president of the Royal Geographical Society. Although they were still unable to prove without doubt that this river was the Nile, it was the closest any explorer had got to settling the great geographical question of the age.