Complex sentence: The expedition left Zanzibar on 25 February and arrived at Banana at the mouth of the Congo on 18 March, somewhat unexpectedly because a telegraph cable had broken, and local officials had received no instructions.
Split into simpler sentences: The expedition left Zanzibar on 25 February and arrived at Banana at the mouth of the Congo on 18 March. Their arrival was somewhat unexpected, because a telegraph cable had broken and local officials had received no instructions.