The Atlantic Ocean is the secon-largest of the world's oceanic divisions; with a total area of about 106.4 million square kilometres (41.1 million square miles). It covers approximately one-fifth of the Earth's surface. The first part of its name refers to the Atlas of Greek mythology, making the Atlantic the "Sea of Atlas". The oldest known mention of this name is contained in The Histories of Herodotus around 450 BC (I 202); see also: Atlas Mountains. Before Europeans discovered other oceans, the term "ocean" was synonymous with the waters beyond Western Europe that we now know as the Atlantic and which the Greeks had believed to be a gigantic river encircling the world; see Oceanus.