Rohlfs was the first European to cross Africa from the Mediterranean Sea southwards to the Gulf of Guinea.
Much of north Africa was Islamic, and to pull off his dramatic feats of exploration Rohlfs learned Arabic, wore local dress, and had himself circumcised in order to pass as a Muslim.
More on Rohlfs (1831-1896) at Encyclopedia Britannica and Wikipedia, as well as his writings at Project Gutenburg.