In the 1840s, European missionaries and explorers began to venture into the east African interior. In Britain, an Association for Promoting the Discovery of the Interior Parts of Africa had been formed as early as 1788, and had since merged with the Royal Geographical Society (RGS). In the following years it would play a leading role in the search for the source of the River Nile.

Zanzibar became the usual starting point for journeys into the interior. Here, the European missionaries and explorers paid their respects to Sultan Said, who 'owned' most of the land they would pass through. They equipped their expeditions with supplies and porters, before sailing to Bagamoyo on the mainland. Many explorers followed the established slaving routes into the interior, often employing slave traders to act as guides.

In 1844, the English Church Missionary Society, unable to find any British recruits, sent the German Johann Krapf to east Africa in an early attempt to convert the local people to Christianity. He was joined by his missionary colleague, Johann Rebmann, who arrived in Zanzibar two years later. They travelled widely across the areas now known as southern Kenya and northern Tanzania. In May 1848 Rebmann became the first European to see Kilimanjaro and in December 1849 Krapf was the first European to see Mount Kenya. (See Exploration in east Africa map opposite.)

Meanwhile on Zanzibar the slave trade continued. By the 1850s about 14,000 to 15,000 slaves a year were being imported into Zanzibar from the mainland, providing Sultan Said with a large income from duties. Zanzibar traders pushed even deeper into the interior, reaching what is now northern Zambia. In 1852 a caravan reached Benguela (in present-day Angola) having completely traversed the continent from east to west, while the following year another group reached Linyanti, in the present-day Caprivi Strip of Namibia.

Through the slave caravans Said had become the nominal ruler of a vast commercial empire stretching along the coast from Mozambique to the Somali ports, and inland to the Great Lakes of Nyasa (Malawi), Tanganyika, Nyanza (Victoria) and Turkana. By the end of his reign Zanzibar's empire covered about 2.5 million km2 (1 million square miles), or 10% of the African continent, including the whole of present-day Tanzania, plus sizeable parts of Malawi, Zambia, DRC, Uganda and Kenya. The Arabs had a saying: 'When the flute plays in Zanzibar, they dance on the lakes.' But it was an empire in name only and Said never attempted to conquer or develop the area.