The term continent usually implies the physical geography of a large land mass completely or almost completely surrounded by water at its borders. However the Europe-Asia part of the border is somewhat arbitrary and inconsistent with this definition because of its partial adherence to the Ural and Caucasus Mountains rather than a series of partly joined waterways suggested by cartographer Herman Moll in 1715. These water divides extend with a few relatively small interruptions (compared to the aforementioned mountain ranges) from the Turkish straits running into the Mediterranean Sea to the upper part of the Ob River that drains into the Arctic Ocean. Prior to the adoption of the current convention that includes mountain divides, the border between Europe and Asia had been redefined several times since its first conception in classical antiquity, but always as a series of rivers, seas, and straits that were believed to extend an unknown distance east and north from the Mediterranean Sea without the inclusion of any mountain ranges.