Looking for diamonds, and find sunken treasure.
While searching for diamonds off the African Coast, archaeologists found a little more than diamonds.
In searching the dry sea bed, a treasure of copper, ivory tusks , ingots & cannons were discovered. Believed to be from around the early 1500's.

In its search for diamonds, the company had originally cleared and drained a stretch of seabed, building an earthen wall to keep the water out so
geologists could work.
Noli said one of the geologists saw a few ingots but had no idea what they were. Then the team found what looked like cannon barrels
Its not known what or the ship was, although early speculation believes it was a trading ship on official business from the Spanish government.

the ship went down in the late 1400s or early 1500s, around the time Vasco de Gama and Christopher Columbus were plying the waters of the New
World.
National Geographic Article
One thing that has many puzzled, is why so much loot with the ivory tusks?
A ship fully laden with cannons was not uncommon for the time, but if they were on a mission to buy tusks, etc. Why would they still have so much
'spending money left' when they went down? Pirates maybe?