The article analyzes, against the background of other ancient authors, how the Roman scholar Pliny the Elder (second half of the 1st century AD) imagines in geographical descriptions in his Historia Naturalis the ocean surrounding the ecumene and the possibility to sail along the ocean coast. The description of Pliny's ecumene, based on the principle of the periplus, implied a continuous narration not only about the shores of ‘Our sea' (which included, besides the Mediterranean, also the Black Sea and the Sea of Azov), but also about the oceanic coasts of the Atlantic, Northern, Eastern, Indian and African Oceans (in terms of the ancient geographers). While the Atlantic coast (from North Africa to Scandinavia) and the Indian coast (from India and Sri Lanka to Somalia and Egypt), the most explored and accessible for navigation, are described in great detail, Pliny also had to conjecture about the rest of the ecumene — washed by the ocean, but never traversed by ancient seafarers. Thus, the resulting picture is a mixture of real knowledge and fantastic speculation. In the article we also analyze data about oceanic voyages found in the works of other ancient authors, who worked as scholars or even as belletrists.