Diogo Cao, Portuguese sea captain, joined the household of Crown Prince Joao, who was responsible for Guinea affairs from 1474. Cao distinguished himself in 1480 by capturing three Spannish ships off the Guinea coast. After the prince succeeded to the Portuguese throne in 1481 as King Joao II he planned further exploration along the African west coast, which was then known down to Cape Santa Catarina, in 1º 53' South latitude. Cao was chosen to command the expedition and departed from Portugal probably in June 1482. After proceeding some distance along the as yet undescribed coast of Africa he found the mouth of a huge river which pushed fresh water far into the ocean. This came to be known as the Congo River. The expedition sailed up the river for some distance and made contact with the local inhabitants, whose king, Manikongo, lived further inland. Cao sent a few black Christian emissaries to the king before continuing his journey. In April 1483 he erected the first of two inscribed stone crosses, on the southern headland at the mouth of the river. Sailing further south the expedition reached what is now Cabo de Santa Maria (13º 26' S) on the coast of Angola, on 28 August 1483. Here a second cross was erected. Remnants of both crosses were recovered during the nineteenth century. As his emissaries had not returned when he reached the Congo River on the return journey he took four Congolese hostages with him to Portugal. He was back home by 8 April 1484, for on that day the king raised him to the nobility and granted him a pension.
In 1485 Cao set out on a second voyage, probably with three ships. The expedition sailed some 140 km up the Congo River and at Yellala, near present Matadi, left inscriptions on the rocks. There they exchanged the Congolese hostages for the Portuguese emissaries. The expedition then proceeded southwards along the coast past their previous point of return and in January 1486 left a stone cross at Monte Negro, latitude 15º 42' South. Though it was later recovered the inscription was illegible. Just south of this point they entered the bay of Porto Alexandre where they found a few native huts. The coast became inhospitable, with bare sand hills and a rocky shore. The expedition reached as far south as present Cape Cross, Namibia, in 21º 47' South latitude, which they named Cabo do Padrao. Here they left their last stone cross, which was recovered and taken to Germany in 1893, still well preserved. Beyond Cabo do Padrao lay a range of hills which were named the Serra Parda - perhaps the Spitzkuppe and Erongo. According to an inscription on a map by Henricus Martellus, dating from 1489, this is where Cao died. No contemporary accounts of his voyages have survived and his progress was reconstructed from later sources, including the inscriptions on the stone crosses he erected.
Cao explored some 2500 km of coastline, mainly along the coasts of Angola and Namibia, as well as the lower Congo River, and brought back knowledge of a highly organised kingdom in west central Africa. His observations, and the experience of his men, contributed much to the success of subsequent exploration of the coast of southern Africa by Bartholomeu Dias*, who departed soon after Cao's expedition returned to Portugal.