In Antiquity, the Mediterranean Sea lay in the centre of the civilised world. It was an essential route of travel and transportation, connecting all major regions of the Greco-Roman world. This situation changed in the late fifth century, when the collapse of the Roman Empire in the West stimulated an ongoing process of regionalisation. With the Arab expansion in the seventh century, core Mediterranean regions of the ancient Empire became occupied by an enemy people, leading Henri Pirenne to assume that it was this event that brought about the end of the ancient Roman world. This paper focuses on these changes in the Mediterranean region, drawing on the testimonies of Jordanes, Paul the Deacon and Liutprand of Cremona. It analyses the terminology used to refer to the sea to retrace contemporary knowledge and perceptions of the wide Ocean and the smaller seas, including the Mediterranean. The paper also discusses contemporary perceptions of the sea and its significance. The evidence points to a gradual estrangement from the sea and growing sense that it represented a threat, ideas that emerged concurrently with a process of regionalisation which is also perceptible in the maritime world, for instance in the lack of a proper name for the Mediterranean as a whole. Scattered indications confirm that the sea was increasingly perceived as a fearsome entity.