The 15th Century.
Sourcing from the Wikipedia, it states,
European exploration of Sub-Saharan Africa begins with the Age of Discovery in the 15th century, pioneered by Portugal under Henry the Navigator. The Cape of Good Hope was first reached by Bartolomeu Dias on 12 March 1488, opening the important sea route to India and the Far East, but European exploration of Africa itself remained very limited during the 16th and 17th centuries. The European powers were content to establish trading posts along the coast while they were actively exploring and colonizing the New World. Exploration of the interior of Africa was thus mostly left to the Arab slave traders, who in tandem with the Muslim conquest of the Sudan established far-reaching networks and supported the economy of a number of Sahelian kingdoms during the 15th to 18th centuries.
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