We know that Romans went to the north areas of subsaharan Africa from the Mediterranean coast of northern Africa, but some academics think that they probably reached also the Gulf of Guinea by sea and/or by crossing the Sahara. I am going to research this possibility.
When Romans conquered Carthage in the second century BC, they also obtained their huge commerce in the Atlantic coast of west Africa.
Indeed a sea-borne merchant people like the parent Phoenicians, the Carthaginians ranged far beyond the Straits of Gibraltar to trade with the Britons and with the Berbers of western Morocco. Sometime before 400 BC Hanno, a Carthaginian admiral, embarked with a fleet of sixty vessels on a famous voyage of exploration down the Atlantic coast of Africa.
He certainly reached the Gulf of Guinea and, from his report of gorillas, probably he arrived to the shores of modern Gabon, but he did not circumnavigate the continent as some have claimed. Furthermore the discovery, two centuries ago, of a cache of Carthaginian coins of the fourth century BC in the Azores—a third of the way across the Atlantic from Portugal—raises the question whether some stray Punic navigator may not even have discovered the New World.
It is noteworthy to pinpoint that the earliest recorded contact with the island of Mogador (near the coast of actual Western Sahara in southern Morocco) was by the Carthaginian navigator Hanno, who visited and established a trading post in the area in the fifth century BC. In the first century BC roman merchants settled in the island and established a small fortified settlement, which may have been the starting point for Roman merchants sailing to the Cape Verde islands and the Gulf of Guinea.
However Romans later conquered Carthago and maintained all trade done across the Sahara after the second century BC and until the fifth century AD. Indeed one roman named Iulius Maternus travelled the most south from the Mediterranean shores inside central Africa.
According to Marinus of Tyre (Ptol. 1,8,5), the roman Iulius Maternus together with the king of the Garamantes set off from Garama (near the Tibesti mountains in the south of actual Libya) to the south and, after four months and 14 days (Ptol. 1,11,5), reached the Ethiopian land of Agysimba, where they saw a great number of rhinoceros (cf. also Ptol. 1,7,2; 8,2; 6 f.; 9,8; 10,1; 11,4; 12,2; 4,8,5; 7,5,2). Maternus seems to have travelled as a trader between AD 83 and 92 AD. To our knowledge, he penetrated further than any other Roman into the African interior and probably reached the gulf of Guinea.
The landscape Agisymba embraced a vast area south of the Sahara from Lake Chad to the west to the Niger bend (and perhaps the delta) and belonged politically to the reign of the king of the Garamantes.This king had his headquarters in the city of Garama in today's Fezzan (western Libya). Agisymba is first time mentioned in the geographical work of the Alexandrian scholar Claudius Ptolemy (2nd century A. D.). An accurate localization of the landscape of Agisymba is still expected, but it is also believed in modern research to be an antecedent kingdom of Kanem. In the second half of the 1st century A D, Iulius Maternus who was probably a native of Roman North Africa, traveled from Leptis Magna to Garama.
There he joined the entourage of the king of the Garamantes and traveled further four months a southerly direction until the landscape Agisymba. There are various considerations, what role Iulius Maternus had played during this expedition: he was seen as a Roman general, as a businessman or as a diplomat. Raffael Joorde -a german historian- wrote that Maternus was a diplomat who received the unique opportunity to make the extensive territory south of the Sahara accessible for the geographers in the Roman world probably reaching the Atlantic ocean in actual Nigeria. The cause of this long journey is assumed by many researchers to be a military campaign of the king of the Garamantes against rebellious subjects.
However some historians (like Susan Raven) believe that there was even another Roman expedition to sub-saharan central Africa: the one of Valerius Festus, that could have reached the equatorial Africa thanks to the Niger river.
Indeed Pliny wrote that in 70 AD a legatus of the Legio III Augusta named Festus repeated the Balbus expedition toward the Niger river. He went to the eastern Hoggar Mountains and the entered the Air Mountains as far as the Gadoufaoua plain. Gadoufaoua (Touareg for “the place where camels fear to go”) is a site in the Tenere desert of Niger known for its extensive fossil graveyard, where remains of Sarcosuchus imperator, popularly known as SuperCroc, have been found). Festus finally arrived in the area in which Timbuktu is now located.
Some academics, such as Fage, think that he only reached the Ghat region in southern Libya, near the border with southern Algeria and Niger. However, it is possible that a few of his legionaries reached as far as the Niger river and went down to the equatorial forests navigating the river to the delta estuary in what is now southern Nigeria. Something similar may have occurred in the exploration of the Nile done under Emperor Nero in Uganda.
After the third/fourth century the roman contacts with sub-saharan Africa started to disappear, because of the final crisis in the roman empire
Maritime travels
Even maritime contacts happened in the western coast of Africa: there are some academic discussions about the possibility of further Roman travels toward Guinea and Nigeria and even the equatorial areas of the Gulf of Guinea.
XIX century map showing the Fernando Po island in the Gulf of Guinea. This island was known to the Romans as one of the "Hesperides": they knew that it was located at 40 days of navigation from the Cape Verde islands (called in roman times "Gorgades")
Indeed according to Pliny the Elder and his citation by Gaius Julius Solinus, the sea voyaging time crossing from the Gorgades (Cape Verde islands) to the islands of the Ladies of the West ("Hesperides") now known as São Tomé and Príncipe and Fernando Po was around 40 days (meaning that Romans knew the exact time needed to reach these equatorial islands -located in front of Camerun/Niger delta- and only with their direct exploration/navigation they could have know this precise time).
Furthermore, a Roman coin -in a good condition- of the emperor Trajan has been found in Congo (http://www.strangehistory.net/2015/02/10/roman-coin-congo/).
Roman coins have been also found in Nigeria and Niger; and in Guinea, Togo and Ghana too. However, it is much more likely that all these coins were introduced at a much later date than that when there was direct Roman intercourse so far down the western coast. But it is possible -even if with a possibility of a very minimal percentage of only 5%, according to researchers- that Roman merchants left there those coins doing their trade when reached the gulf of Guinea.
Finally it is important to remember that Augustus, based on the discovery of a sunken roman merchant ship from southern Hispania in the Djibouti area (in the horn of eastern Africa), wanted to organize around Africa a roman maritime expedition (to be done initially by his adoptive son Gaius Caesar when he sailed from Egypt's Berenice toward Aden).
It was going to be done -around 2 AD- from southern Egypt to Mogador and Sala (in actual Morocco). But it seems that it never took place.
