Explorers of the Age of Discovery, from the late 15th century on, proved that Africa was almost entirely surrounded by sea, and that the Indian Ocean was accessible from both west and east. These discoveries reduced the area where the continent could be found; however, many cartographers held to Aristotle's opinion. Scientists such as Gerardus Mercator (1569) and Alexander Dalrymple as late as 1767 argued for its existence, with such arguments as that there should be a large landmass in the south as a counterweight to the known landmasses in the Northern Hemisphere. As new lands were discovered, they were often assumed to be parts of the hypothetical continent.