The Iapetus Ocean was an ocean that existed in the Neoproterozoic and Paleozoic eras of the geologic timescale (between 600 and 400 million years ago). The Iapetus Ocean was situated in the southern hemisphere, between the paleocontinents of Laurentia, Baltica and Avalonia. The ocean disappeared with the Caledonian, Taconic and Acadian orogenies, when these three continents joined to form one big landmass called Laurussia.
Because the Iapetus Ocean was positioned between continental masses that would at a much later time roughly form the opposite shores of the Atlantic Ocean, it can be seen as a sort of precursor of the Atlantic. The Iapetus Ocean was therefore named for the titan Iapetus, who in Greek mythology was the father of Atlas.